Scotland Yard is investigating allegations that detectives working for its anti-corruption unit have been paid thousands of pounds by a firm of private investigators.

A parliamentary inquiry was told today that invoices, also seen by the Guardian, purport to show how a firm of private investigators made payments in return for information about the Metropolitan police investigation into James Ibori, a notorious Nigerian fraudster.

On Tuesday, the Commons home affairs select committee was told by a lawyer involved in the case that invoices showed about £20,000 of potential payments to police officers in what amounted to an undetected case of "apparent corruption right at the heart of Scotland Yard".

In recent weeks, as the Guardian investigated the allegations, the Met has sought to discourage the paper from publishing details about the case. But , after MPs heard the evidence, the Met dropped its previous insistence that there was "evidence that casts doubt on the credibility" of the allegations.

A police source with knowledge of the investigation, which has been ongoing since October, said developments over the last 24 hours had now led police to take the allegations more seriously.

The case revolves around a private investigation firm called RISC Management. Five years ago the firm was hired to work for Ibori, a former Nigerian state governor, after he discovered he was being investigated by the Met for serious fraud. Ibori recently pleaded guilty to money laundering and was jailed in the UK, after the conclusion of a major investigation into his financial affairs.

The allegation now being investigated by police is that some detectives on the Met's Proceeds of Corruption Unit, which investigated Ibori, were receiving payments in exchange for information about the ongoing investigation.

Invoices and other documents appearing to support the allegations have been anonymously posted to the Met and Independent Police Complaints Commission (IPCC).

The documents have also been seen by the Guardian and separately sent to the home affairs committee, which is conducting an inquiry in whether private investigators should be subject to statutory regulation.

Keith Vaz, the chair of the committee, has said there is growing concern in the wake of the phone-hacking scandal that some private investigators are operating in "the shadows" of the law. The Commons inquiry has been scrutinising the nexus between private investigators - many of whom are retired police officers - and their former colleagues who are still serving.

On Tuesday morning, Mike Schwarz, a lawyer who represents one of Ibori's co-accused, told the inquiry about what he understood to be the significance of the material.

He said it indicated possible corruption at the heart of the police investigation into the Nigerian politician's money laundering activities. The invoices are alleged to be from RISC Management to Speechly Bircham, a top firm of lawyers hired by Ibori to prepare his defence.

Schwarz told MPs the invoices "perhaps" documented "payments made by RISC Management to sources, presumably police officers or those close to the investigation". He added: "The records, which I think the committee have, show about half-a-dozen payments totalling about £20,000 over a period of eight or nine months [...] it appears to be inappropriate if not corrupt."

Schwarz told the committee that he believed RISC Management had been hired to "extract" information from the police investigation into Ibori. He said he had also seen emails - which he believed had also been forwarded to the committee - which confirmed "contact" between detectives investigating Ibori and the private investigators.

Schwarz, from Bindmans solicitors, represents Bhadresh Gohil, a London-based solicitor jailed along with Ibori for orchestrating his money laundering scam. Gohil is now considering an appeal. Gohil is understood to have been sent the invoices, anonymously, while in Wandsworth Prison last summer.

In a statement, the Met said: "The [force] is investigating an allegation that illegal payments were made to police officers for information by a private investigation agency. The Directorate of Professional Standards (DPS) referred the matter to the Independent Police Complaints Commission in October 2011 which agreed to supervise a DPS investigation into the allegations."

Following Schwarz's evidence to parliament, the Met said it had dropped its previous claim to have recently "uncovered evidence" casting doubt on the allegations. Previously, the force had suggested an active line of inquiry was the theory that Gohil or his associates had fabricated the allegations to undermine the prosecution. In a previous statement, provided on Friday, the force said: "As a result of inquiries police have uncovered evidence that casts doubt on the credibility of these allegations. Warrants have been executed at two addresses in London and a quantity of paperwork and computer equipment recovered."

Two weeks ago, following raids on properties, one of which was the Gohil's family home in Kent, the force said: "Officers believe that they have identified the originator of the information and a line of enquiry suggests that there may have been an attempt to pervert the course of justice."

However, sources at the Yard said previous statements no longer fully represented their position. A source with knowledge of the Met inquiry said the change of stance was unrelated to Schwarz's parliamentary evidence. The source said that, instead, there had been developments in the investigation over the last 24 hours.

Schwarz named three serving Met police officers in his testimony to parliament as being potential "culprits": detective inspector Gary Walters, detective constables named as John MacDonald and "Clark". All three officers declined an opportunity to respond to the allegations when contacted by the Guardian last week. However, RISC Management indicated Walters would deny "any and all allegations" [see footnote].

RISC Management denied all the allegations about the company, saying it was not aware of the Scotland Yard investigation and had no knowledge of the alleged financial records. The firm confirmed it had been hired by Ibori's lawyers but denied making corrupt payments, saying it "has never paid a serving police officer for information and would never approve such payments".

Keith Hunter, chief executive of the company, said: "RISC management does not need to pay serving police officers for confidential information as we pride ourselves on our ability to provide positive solutions and accurate information legitimately. RISC Management has a highly respected reputation for conducting professional investigations".

He added that his company was "proud to have a network of highly professional consultants, contacts and resources. These individuals are hired precisely because of their unique skill set and expertise".

He accused Schwarz of "grandstanding" in front of the Commons committee, instead of taking the "correct course of reporting the matter to the police". He said Schwarz had not produced any evidence to support his claims and acted for a convicted solicitor, Gohil, who was jailed for seven years for money laundering.

Speechly Bircham denied any knowledge of wrongdoing and said it would be willing to assist with any police inquiries. The law firm stressed Schwarz did not suggest in his evidence to parliament that Speechly Bircham was "party to illegal or corrupt payments" and said any such allegation would be false and defamatory.

Ian Timlin, the former Speechly Bircham lawyer who was at the time representing Ibori, said neither he nor the firm had "any knowledge of any payments to police officers for information." He added: "At no time, did RISC ever inform me who or what was the source/s they were paying."

• This footnote was added on 11 June 2012. Former ­detective inspector ­Gary Walters points out that he was not ­directly contacted by a Guardian journalist and he denies any wrongdoing.