Scotland Yard's former director of internal audit has made serious allegations about ex-commissioners Lord Blair and Sir Paul Stephenson, the Leveson Inquiry heard today.
Peter Tickner made the as-yet undisclosed claims in a written statement which is due to be made public when he gives evidence to the press standards inquiry tomorrow.
But Neil Garnham QC, representing the Metropolitan Police, complained that the charges were "unproven" and suggested they were an attempt to "settle old scores".
Mr Garnham also raised concerns about allegations against senior officers made in a statement to the inquiry by Detective Chief Inspector Clive Driscoll, who led the Met's investigation into the racist murder of Stephen Lawrence.
Lord Justice Leveson said he would consider postponing Mr Tickner's evidence and suggested that the Mayor's Office for Policing and Crime (MOPC) - Scotland Yard's new governing body - could investigate his claims further.
Mr Garnham said: "The allegations being made against people like Lord Blair, Sir Paul Stephenson and others are very serious.
"They come, to use the popular expression, from leftfield. They have not been prefaced or anticipated before."
He added: "Given the nature of these allegations, they are certain to receive significant publicity... The allegations being made are unproven and unsupported by independent evidence.
"They have, we would say, the flavour of attempts to use the inquiry as a vehicle to settle old scores, and those criticised have had no chance to deal with the issues when they gave evidence.
"These previous witnesses face being traduced in the press without any possibility of effective redress or rebuttal."
Lord Justice Leveson proposed that the MOPC, which replaced the Metropolitan Police Authority in January, could look into Mr Tickner's allegations.
He said: "It may be that equally the authority or its successor body ought to be considering that which its former employee (Mr Tickner) has said, to find out whether there is material which I ought to know about that would either utterly undermine that which he's said, in which case I may take a view about whether it should be called, or alternatively a different line is taken, in which case I have to make all sorts of arrangements to make sure that I have been fair to everybody concerned."
Mr Tickner could not immediately be contacted for comment.
But his website says he investigated the use of corporate cards and expenses of senior police officers while he was the Met's director of internal audit from 1995 until he took early retirement in 2009.
He also set up a unit to investigate contractor and staff fraud and corruption at Scotland Yard, according to his website.
Mr Driscoll's evidence relates to "potentially damaging leaks" from the Lawrence investigation, the Evening Standard reported.
Met Assistant Commissioner Cressida Dick told the Leveson Inquiry on Monday that she did not discuss the case at meetings of Scotland Yard's top officers to minimise the risk of unauthorised disclosures of information to journalists.
She said: "I took the decision when I took this on that, as soon as we started our forensic review in 2005-06, I would personally only brief the Commissioner, and only intermittently, on the progress of that review.
"Because I was absolutely determined, if I could possibly ensure it, that only those people who really needed to know did know in case there was any unhelpful media coverage which might undermine the investigation or any future trial in terms of people's right to a fair trial."
The inquiry heard that there was a leak about the Lawrence case in November 2007, when the Daily Mail revealed that police were investigating new forensic evidence.
Ms Dick launched a leaks investigation but it found no proof that anyone from the Met had passed any information to a journalist.