In recent years there has been growing concern that officers may be using the powers without sufficient grounds, fears some say are justified by low levels of arrests and further action arising from stops.
Official figures show young black men are more likely to be stopped than their white counterparts, raising further concerns that some officers are targeting specific communities without justification.
In a statement to MPs, Mrs May said where officers “misused” their power it wasted police time and was counter-productive.
She added: “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.”
In London, 80% of stops carried out in 2011 resulted in no further action, a rate criticised by Metropolitan Police Commissioner Sir Bernard Hogan-Howe when the took over the force.
Reforms introduced by Sir Bernard have seen the number of stops carried out fall by a third while almost doubling the number of arrests resulting from them.
Mrs May said the Met and Sir Bernard had proven it was possible to use the powers more fairly while still cutting crime and keeping neighbourhoods safe.
She warned that up to a quarter of a million stops in England and Wales “could have been illegal” after a Her Majesty’s Inspectorate of Constabulary review found 27% of records didn’t contain reasonable grounds to search people.
In order to ensure all stops are justified in the future, Mrs May said the police code of practice would be revised to clarify what constitutes “reasonable grounds for suspicion” and that officers who misuse their powers risk formal performance or disciplinary proceedings.
Mrs May said the College of Policing would assess individual officers’ fitness to use stop and search powers and those who “do not understand the law” will be barred from carrying out stops.
In addition, stop and search statistics will be uploaded to crime maps, allowing local communities to monitor use of the powers and hold forces and local police and crime commissioners to account.
Speaking after Mrs May’s statement, the Met’s Commander Adrian Hanstock said: “We have made significant improvements 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 and this work has been acknowledged by the Home Secretary.
“The MPS is 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.
“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.”
