UK's biggest force signs up to code of conduct after discovery that quarter of searches last year may have been illegal

The UK's biggest police force has pledged to publish data on all outcomes of controversial stop and search measures by its officers under a Home Office scheme.

The Metropolitan police service, which employs more than 31,000 officers, has signed up to the "best use of stop and search code of conduct", previously announced by the home secretary, Theresa May, who has admitted the power was being misused.

The changes are being brought in after Her Majesty's Inspectorate of Constabulary (HMIC) found that 27% of stop and searches did not contain reasonable grounds for suspicion, meaning more than 250,000 of the 1m searches conducted last year could have been illegal. In addition, black and minority ethnic members of the public were up to six times more likely to be searched than white people.

Under the scheme, forces who sign up voluntarily will agree to more limits on blanket Section 60 stops, used on the anticipation of serious violence without suspicion a person is carrying weapons, while better records will be kept of each instance and published online. The Met said it was introducing those two elements of the new plan from Tuesday and comes after Section 60 searches were used as a tactic in combating violent crime at this year's Notting Hill carnival.

Last year, 47,141 arrests were made as a result of stop and search, representing 20% of total arrests by the Met, and of the 251,161 people who were stopped and searched from August 2013 to July 2014, 115,270 (46%) were white, 72,016 (29%) were black and 34,267 (14%) were Asian. Men accounted for 94% of all searches.

Cdr Adrian Hanstock, the force's lead for stop and search, said: "The best use stop and search scheme supports the Met's ongoing drive to make stop and search more intelligence led and effective.

"The Met has made significant improvements to stop and search over the last two years to not only reduce the total number of people we search, but also to ensure that our officers focus on those areas and types of crime that the public are most concerned about.

"As a result, we are arresting more people than ever before following the more targeted use of stop and search and routinely find people in possession of weapons and stolen items.

"We recognise the impact stop and search can have on individuals and communities. Our work with communities and monitoring groups is helping to ensure we are more transparent than ever in how stop and search helps to reduce crime and keep people safe, but we of course recognise there is much more to do to improve confidence across all communities in the use of the powers."

Speaking in the House of Commons when announcing the shakeup in April, May said misuse of the power damaged public trust in the police. "While it is undoubtedly an important police power, when it is misused stop and search can be counterproductive.

First, it can be an enormous waste of police time. Second, when innocent people are stopped and searched for no good reason, it is hugely damaging to the relationship between the police and the public. In those circumstances it is an unacceptable affront to justice."