The advert warns people with information about gangsters they could have "blood on their hands" if they fail to tell police.

Officers believe some people are too scared to come forward or do not want to "grass up" friends.

But the advert focuses on the guilt people will feel if they do nothing about gun crime.

The advert's launch on Monday coincides with the fifth anniversary of Scotland Yard's Operation Trident, which was launched to reduce gun crime in London's black community.

There were 162 Trident-related shootings in the capital last year and in the 12 months up to the end of March this year, 18 people were shot dead.

The advert will be shown in cinemas around London, particularly in those boroughs worst affected by gun crime.

Detective Chief Superintendent John Coles, the head of Trident, said: "It is about getting people with the information to tell us about it and not for them, after something happens, to end up in a situation where they feel guilty.

"If they know about something that could have prevented a shooting and they did not pass that on to us, in a way they could end up with 'blood on their hands' as well.

"It is a fantastic advert. Hopefully it will make a lot of people sit up and take notice."

Mr Coles added: "This is not just about targeting the people who are directly involved, it is also about raising awareness among others that this is going on.

"People do ignore it, they say it is the black community's problem - but it is not, it is society's problem."

The advert will be unveiled at a launch at the Ritzy Cinema in Coldharbour Lane, Brixton, south London.