London mayor Ken Livingstone denied involvement in reporting Tory rival Boris Johnson to police last night over the alleged "illegal removal" of a cigar case from Iraq.

Mr Johnson, who is the MP for Henley, has blamed the Scotland Yard inquiry on a concerted effort by his political opponents and branded it a waste of police time.

He publicly revealed several years ago that he had removed the case as a souvenir from the home of Iraqi former deputy prime minister Tariq Aziz and had offered to return it.

But he has now received a letter from detectives saying their attention had been drawn to the fact that there was "in your possession an item that may be Iraqi cultural property".

"Obviously, it is at an early stage of our enquiries but I would offer to act as the 'constable' should you wish to transfer the item into our possession for further investigation," it went on.

Mr Johnson, who has repeated his offer to hand over the cigar case, said: "There were over 18,000 crimes in London last month and yet the police write to me about this?

"What this shows is a concerted effort by my political opponents to waste police time by dragging up an article that I wrote five years ago and trying to make political mileage out of it.

"When knife crime is on the rise in our capital city, can it be right that police time is allowed to be wasted in this way?"

The Livingstone campaign said it was not behind the report to the police.

"We learnt of it from the newspapers and we consider the issue ridiculous and do not support it," a spokesman said.