The lawyer for the family of the Brazilian man shot dead on the London Tube has appealed for openness in the inquiry into the case.
Mr Menezes was killed after being mistaken for a suicide bomber
Gareth Peirce said the independent investigators had been "kept at bay by the police for days".
It follows a claim in the News of the World that the inquiry report says some police evidence was rewritten.
Mr Menezes was shot dead by mistake by police at Stockwell Tube station, the day after the failed 21 July bombings.
A Brazilian government delegation is due in the UK on Monday for talks.
On Sunday, the de Menezes family asked to see the Independent Police Complaints Commission report, after the paper ran what it said were leaked details.
The Metropolitan Police said it would not be appropriate to comment and it is a matter for the IPCC.
The IPCC said it was not commenting on the claims.
Ms Pierce told the BBC the allegation of a leak was a "terrible allegation in an already terrible case".
"We know there is a sorry history in this country of potential of police fabrication of evidence," she said.
"That's exactly why an independent investigative body was created to oversee allegations of police crimes.
"We don't know what happened, but we do know there was, regrettably, every opportunity for it to happen.
"It's fundamental to any investigation that primary evidence should be seized and preserved before it could be interfered with.
"However, the independent body was kept at bay by the police for days, on the grounds that it was in the interests of national security."
It is the second alleged leak from the IPCC probe. In August 2005, leaked documents appeared to contradict the original official account of how police came to shoot the 27-year-old electrician.
Ms Pierce said the family had called for transparency in the investigation and it was important to make sure that concern about prejudicing any trial or national security did not cover up "police misdeeds".
A report on the IPCC investigation into Mr de Menezes' death went to the Crown Prosecution Service on 19 January.
Copies have also been sent to Scotland Yard, the Metropolitan Police Authority and Inner South London coroner John Sampson.
But, due to what the IPCC calls "legal considerations" it has not gone to the family, in a move they describe as unacceptable.
The report will not be made public until any trial or inquest arising from the case has been completed.
Three senior officials from the Brazilian government and judiciary are in London this week to find out when and how the family will learn more.
Ambassador Manoel Gomes Pereira and fellow delegation members Wagner Goncalves and Marcio Pereira Pinto Garcia will visit until February 2.
The IPCC said it had offered to meet them but had made it clear that it cannot tell them anything the family does not already know.
But, a spokesman said, it will explain the next steps in the process.
A separate IPCC investigation is being held into Metropolitan Police Commissioner Sir Ian Blair's handling of the affair.
In comments published in the Guardian on Monday, Sir Ian said the Met had made "a serious mistake" by not correcting reports that Mr Menezes' clothing and behaviour had been suspicious.
"Clearly the Met could have taken the decision on the Saturday when we recognised that we had killed an innocent man, we could have put the record straight," he told the newspaper.
"Although we did put the record straight by saying he wasn't connected, we didn't put the record straight about all the issues around him, jumping over barriers and heavy coats and so on"