British police are investigating a money-laundering allegation related to a big oil field bought by Shell and ENI from Nigeria for $1.3 billion, after most of the cash they paid allegedly ended up in a company linked to a former oil minister, report Reuters.

The probe concerns off-shore block OPL 245, which industry sources say contains up to 9.23 billion barrels of crude – more than enough to keep China running for two and a half years – the ownership of which had been in dispute for more than a decade.

“The crime unit is investigating a money-laundering allegation in the UK in connection with OPL 245. The investigation is an early stage,” a UK spokesman said. Transparency campaigners, who asked the UK to look into the matter, assert that Shell and ENI used the Nigerian government as a go-between to obscure the fact that they were dealing with the former oil minister, who also has a 2007 money-laundering conviction in France related to bribes he was alleged to have taken when in government.

 In his capacity as oil minister, Reuter said he awarded block OPL 245 in 1998 for a payment of just $2 million to Malabu Oil and Gas, a company in which is alleged he played a prominent role.

The critics claim that Shell and ENI, which haven’t been accused of any legal wrongdoing, wanted to distance themselves from the minister given his reputation and his involvement in the original award of the oil block to Malabu.

A Shell spokesman told Reuters it had purchased the block from the government, making no payment to Malabu, and that it acted transparency and in accordance with Nigerian law.

ENI declined to comment to Reuters, but it told shareholders in May that the transaction was with the government, not Malabu.

Reuters was not able to locate the minister for comment. His lawyer did not immediately respond to a request for comment, Reuters said.

While Shell and ENI say they bought the block from the Nigerian government, for which they paid it $1.3 billion in 2011, Nigeria says it was helping resolve an ownership dispute over the block between Shell and Malabu and immediately transferred $1.09 billion from the sale to Malabu. The government retained the remainder.

The minister had awarded the block to Malabu during the rule of military dictator Sani Abacha, whose son Mohammed and other close allies were shareholders in the company. That deal was later annulled after the death of Abacha by a new government that judged the award improper.