Diezani Alison-Madueke, 65, was found not guilty after a trial at London's Southwark Crown Court of five counts of accepting bribes and a charge of conspiracy to commit bribery.
Alison-Madueke was Nigeria's oil minister between 2010 and 2015 and the first female president of the oil exporters group Opec.
The verdict is a blow for the UK's National Crime Agency (NCA), which had been investigating one of Africa's most prominent political figures for 13 years.
From the start of the trial in January, defence lawyers questioned the fairness of the prosecution's case, suggesting vital documents showing Alison-Madueke's innocence had gone missing in Nigeria.
They also said the long delay in bringing the case to court was unjust and a sign of Britain's "broken criminal justice system".
Also cleared by the jury were Alison-Madueke's older brother Doye Agama, 69, an archbishop at a Pentecostal church in Manchester, who was acquitted of conspiracy to commit bribery.
Oil industry executive Olatimbo Ayinde, 54, was found not guilty of bribery and bribery of a foreign public official.
She had faced prosecution despite being an informant in an anti-corruption probe by the Nigerian authorities.
'Madam due process'
Alison-Madueke portrayed herself in court as a role model for women, a tireless fighter against corruption, and someone who was such a stickler for the rules she was nicknamed "Madam due process".
She became the first female member on the Nigerian board of oil and gas giant Shell in 2006, and four years later was appointed oil minister, the country's second most senior politician. She became president of Opec in 2014.
"In a very patriarchal society, to have a woman sitting at the helm was a major no-no," she told the court, suggesting this had made her a target for unnamed male opponents.
Prosecutor Alexandra Healy KC said the former minister improperly allowed powerful men with lucrative government contracts in the oil business to bankroll her extravagant lifestyle.
Six of them were named on the indictment, although none were charged.
But the prosecution failed to provide evidence she awarded contracts to any of the oil tycoons named because of bribes.
"At no time did I ask, take, or seek a bribe or bribes of any sort," Alison-Madueke told the court, saying many of the luxury items purchased were not for her, and that she had been with the oil men to offer advice on interior design in their own properties.
Alison-Madueke told the court that Nigerian ministers were not allowed to hold foreign bank accounts when on service overseas, and her department's office in London was in such disarray that she relied on wealthy businessmen funding her living expenses.
She said they were always reimbursed in Nigeria and evidence proving this had been seized from her home in Abuja but never produced by the authorities there.
Former Nigerian president Goodluck Jonathan, who had appointed Alison-Madueke, did not appear as a witness. But he provided a statement in which he said third parties would often pay for transport, accommodation and other items for ministers on official overseas business.
Questions raised by the case
There were a number of unanswered questions that seem to have fatally undermined the prosecution.
Defence barrister Jonathan Laidlaw KC questioned why the Nigerian government had not sought to prosecute Alison-Madueke.
He said Alison-Madueke had "effectively been kept prisoner in this country for almost 11 years… unable to work, unable to travel" while the NCA had "done nothing to bring about the extradition" of the six oil men said to have paid bribes to her.
The jury was not told why they had not been charged.
The defence claimed the investigation had been compromised from the start because the NCA was denied access to the search of Alison-Madueke's Abuja home in 2015.
They relied on work done on their behalf by Nigeria's Economic and Financial Crimes Commission (EFCC).
But while the prosecution told the jury to trust how the EFCC had gathered evidence against Alison-Madueke, at the same time it advised them to disregard the commission's evidence in relation to a co-defendant in the trial.
The defence case of Olatimbo Ayinde, the oil industry executive who was also found not guilty by the jury, was she had been working as an informant for the Nigerian authorities to expose corruption.
Ayinde, a Nigerian businesswoman with British citizenship, said she been encouraged by the west African country's security services to "play along" with those in government who were asking her for bribes.
An EFCC investigator, who had travelled to London from Nigeria, told the court Ayinde had given "vital information that assisted the investigation".
"Miss Ayinde's plan was to help law enforcement and now she's there in the dock," said her barrister Jonathan Lennon KC who had sought unsuccessfully to have his client's case thrown out of court.
In a statement after the verdict, Alison-Madueke said her "nightmare is over".
She said: "For 11 long, gruelling years this case has hung over my head and has tormented me and my family. But today, the past decade of relentless and unjust vilification, condemnation and scrutiny has finally come to an end."
Alison-Madueke portrayed herself in court as a role model for women, a tireless fighter against corruption, and someone who was such a stickler for the rules she was nicknamed "Madam due process".
She became the first female member on the Nigerian board of oil and gas giant Shell in 2006, and four years later was appointed oil minister, the country's second most senior politician. She became president of Opec in 2014.
"In a very patriarchal society, to have a woman sitting at the helm was a major no-no," she told the court, suggesting this had made her a target for unnamed male opponents.
Prosecutor Alexandra Healy KC said the former minister improperly allowed powerful men with lucrative government contracts in the oil business to bankroll her extravagant lifestyle.
Six of them were named on the indictment, although none were charged.
But the prosecution failed to provide evidence she awarded contracts to any of the oil tycoons named because of bribes.
"At no time did I ask, take, or seek a bribe or bribes of any sort," Alison-Madueke told the court, saying many of the luxury items purchased were not for her, and that she had been with the oil men to offer advice on interior design in their own properties.
Alison-Madueke told the court that Nigerian ministers were not allowed to hold foreign bank accounts when on service overseas, and her department's office in London was in such disarray that she relied on wealthy businessmen funding her living expenses.
She said they were always reimbursed in Nigeria and evidence proving this had been seized from her home in Abuja but never produced by the authorities there.
Former Nigerian president Goodluck Jonathan, who had appointed Alison-Madueke, did not appear as a witness. But he provided a statement in which he said third parties would often pay for transport, accommodation and other items for ministers on official overseas business.
Questions raised by the case
There were a number of unanswered questions that seem to have fatally undermined the prosecution.
Defence barrister Jonathan Laidlaw KC questioned why the Nigerian government had not sought to prosecute Alison-Madueke.
He said Alison-Madueke had "effectively been kept prisoner in this country for almost 11 years… unable to work, unable to travel" while the NCA had "done nothing to bring about the extradition" of the six oil men said to have paid bribes to her.
The jury was not told why they had not been charged.
The defence claimed the investigation had been compromised from the start because the NCA was denied access to the search of Alison-Madueke's Abuja home in 2015.
They relied on work done on their behalf by Nigeria's Economic and Financial Crimes Commission (EFCC).
But while the prosecution told the jury to trust how the EFCC had gathered evidence against Alison-Madueke, at the same time it advised them to disregard the commission's evidence in relation to a co-defendant in the trial.
The defence case of Olatimbo Ayinde, the oil industry executive who was also found not guilty by the jury, was she had been working as an informant for the Nigerian authorities to expose corruption.
Ayinde, a Nigerian businesswoman with British citizenship, said she been encouraged by the west African country's security services to "play along" with those in government who were asking her for bribes.
An EFCC investigator, who had travelled to London from Nigeria, told the court Ayinde had given "vital information that assisted the investigation".
"Miss Ayinde's plan was to help law enforcement and now she's there in the dock," said her barrister Jonathan Lennon KC who had sought unsuccessfully to have his client's case thrown out of court.
In a statement after the verdict, Alison-Madueke said her "nightmare is over".
She said: "For 11 long, gruelling years this case has hung over my head and has tormented me and my family. But today, the past decade of relentless and unjust vilification, condemnation and scrutiny has finally come to an end."
By Steven Swann, BBC
