Boris Johnson said Mr Aziz had subsequently offered the case as a gift
Police have forced London Mayor Boris Johnson to hand over a cigar case belonging to Iraq's former deputy prime minister Tariq Aziz.
The ex-MP, who obtained the red leather case from Mr Aziz's bombed-out home while visiting Iraq as a journalist in 2003, said the situation was "stupid".
He said: "The police have no choice but to investigate this ludicrous affair."
Under the Iraq (UN Sanctions) Order 2003, anyone possessing Iraqi cultural property must give it to the police.
Mr Johnson deposited the case into custody on Monday afternoon, following instructions from the police.
'Wasting police time'
Scotland Yard said it would be held "while further enquiries are made".
Writing in the Daily Telegraph, Mr Johnson said he had obtained Mr Aziz's case a few days after the Americans had captured Baghdad, claiming the circumstances were "so morally ambiguous that I cannot quite think of it as theft".
"As I stared at the remains of his home, I saw utter destruction - surely the looters had left nothing of value?" he said.
"And there, just by my toe, protruding from beneath a piece of dusty plywood, was the cigar case. Actually, it was only the bottom half of a cigar case, in thick red leather and coarsely stitched.
"But I immediately saw its importance. If this was the cigar case of Tariq Aziz, think of the scenes it had witnessed."
The mayor, who says he subsequently received a letter from Mr Aziz offering the case as a gift, claims the police only became involved after "Labour stooges" demanded he be prosecuted when they found an article he had written about the case.
"I briefly toyed with making a fuss, and pointing out how utterly selfish and stupid it was of Labour to waste police time on this kind of thing," he said.
"Just when the police are trying to focus on beating knife crime and making the streets safe, they are told they must lavish money and manpower on a preposterous investigation that will do nothing for the security of the public."
He asked why other politicians, such as Tony Blair who he says "the Commons failed to impeach" over the Iraq war, had not been "brought to book".
"I am informed by my friends in the Metropolitan Police that I am shortly to become the one and only Western politician to be brought to justice for crimes committed in Iraq."
Stephen Pound, Labour MP for Ealing North, said he was "less worried about the cigar case" than he was about London's mayor spending "half a day" writing a column for the Daily Telegraph.
"What he has done is not to deny he's committed the crime but has blamed the person who has brought it to the attention of the police," he said.
"Boris adds to the gaiety of the nation, there's no doubt about that. I would rather have a full time mayor than a part-time journalist running this city."
A police spokesman said: "The Met works very closely with a number of countries, including Iraq, to recover items that are considered culturally significant.
"In order to establish the origin and potential significance of the item, such items must be submitted to police custody for further examination."
Failing to adhere to the 2003 order is considered an offence, unless the person can prove they were not aware that the property was illegally removed from Iraq.