Boris Johnson has criticised a Met Police investigation into alleged war crimes carried out by Israel in Gaza.
Scotland Yard has put up posters at major British airports including Heathrow, appealing for anyone returning from Israel or Gaza who has witnessed war crimes, terrorism or crimes against humanity, to get in touch.
Former prime minister Mr Johnson told The Telegraph he fears it marks a "worrying politicisation of the Met Police”.
The Met said it distributed the posters to support an ongoing investigation by the International Criminal Court (ICC), into events in Israel and Palestine.
It said its War Crimes Team is "obliged to support any investigations opened by the ICC that could involve British subjects", under the terms of the 1998 Rome Statute.
The posters created by counter-terror police are written in Arabic and English, and address "travellers who have been in Israel/Palestinian Territories”.
“If you have been in Israel/Palestinian Territories and have witnessed or been a victim of terrorism, war crimes or crimes against humanity, then you can report this to the UK police," say the posters.
"UK policing is supporting the work of the International Criminal Court, which is investigating alleged war crimes in Israel and Palestine from June 2014 onwards.
“Any evidence gathered may be shared with the ICC in support of their investigation.”
Mr Johnson told The Telegraph on Wednesday: “This sounds like a worrying politicisation of the Met Police.
“When I was mayor of London I made it clear that we would not import foreign wars or disputes onto the streets of London. The Met would be better off fighting knife crime in the capital.”
A former senior Cabinet minister also reportedly told the newspaper: “This begs the question of why the Met Police have got involved. They have a lot of work to do domestically, while what’s happening in Israel and Gaza is a grey area right now.“
The International Centre of Justice for Palestinians (ICJP) in turn criticised Mr Johnson's remarks, saying: "A police investigation should not be subjected to political interference in this way."
A Met Police spokesperson said: "In order to meet our obligations to support the ongoing ICC investigation linked to Israel and Palestine, and due to the fact we were aware that relatively large numbers of people - including many British nationals - were returning on flights to the UK from the region, posters signposting how people could contact the War Crimes Team have been displayed at certain airports to make any potential witnesses or victims aware of how they can get in touch," it added.
There have been allegations of war crimes against both Israel and Hamas, since war broke out almost three months ago.
Israel says 1,200 people were killed and some 240 abducted when Hamas launched a cross-border assault on Israel on October 7.
In response, Israel has since carried out a near-relentless ground and aerial blitz of Gaza, and the total recorded Palestinian death toll had reached 22,313 by Wednesday - almost one per cent of its 2.3 million population, Gaza's Hamas-run health ministry said.
Families of some Hamas victims have filed a war crime complaint with the ICC, calling for the October 7 massacre against Israel to be investigated as "crimes against humanity".
Meanwhile a top United Nations official is among those who have accused both Israel and Hamas of war crimes.
UN human rights chief Volker Turk said: "The atrocities perpetrated by Palestinian armed groups on 7 October were heinous, they were war crimes - as is the continued holding of hostages.But he added: “The collective punishment by Israel of Palestinian civilians is also a war crime, as is unlawful forcible evacuation of civilians.”
