Former defence secretary Sir Michael Fallon has welcomed the Met police investigation into the leak of official communications written by Britain's ambassador to the US Sir Kim Darroch.
Sir Kim quit his post amid diplomatic furore, after memos he wrote which criticised the Trump administration were published.
An investigation was launched by the Foreign Office before Scotland Yard confirmed its own inquiry.
"It looks prima facie as a clear breach of the Official Secrets Act," Sir Michael told the BBC Radio 4 Today programme.
"It has been extremely damaging to our diplomatic efforts, so the sooner we find out who did it and have him investigated and prosecuted the better."
Sir Michael also backed a call by the Metropolitan Police assistant commissioner Neil Basu for any news organisations holding leaked government documents to return them.
"If they are receiving stolen material they should give it back to their rightful owner," he said.
"They should also be aware of the huge damage that has already been done and the potentially even greater damage to be done by further breaches of the Official Secrets Act. That is the law of the land."
He added: "There is clearly enormous political damage to our relationship with our strongest ally, the United States. I think the Government and the police are fully entitled to find out who was involved in that and if they can to prevent it happening again."
However the chairman of the Commons Foreign Affairs Committee, Tom Tugendhat, questioned whether journalists who published such material were committing an offence.
"I doubt it is a crime to publish. The ability to have a free press is essential," he said.
Sir Kim announced on Wednesday he was resigning, stating his position had become "impossible" following the leak of the cables in which he described President Trump's White House as "inept" and "dysfunctional".
His comments drew a furious response from the president who denounced him as a "very stupid guy" and a "pompous fool" and said the the White House would no longer deal with him.
