As many as one in seven stop and searches carried out by police in a year may have been unlawful, a new report indicates.
The police watchdog Her Majesty's Inspectorate of Constabulary (HMIC) examined 100 stop and search records in each of the 43 police forces in England and Wales in the 12 months up to March 2015 and found that some 15% did not have reasonable grounds recorded, as required by law.
Out of the 4,259 records examined, only 17% of the items sought in stop and searches were found, highlighting the low success rate of the tactic, the HMIC's report into police legitimacy found.
HMIC said the findings were "disappointing" and showed that police were failing to meet this "litmus test of legitimacy" - despite the percentage having shrunk from 27% in 2013.
Her Majesty's Inspector of Constabulary Stephen Otter, who led the inspection, said the findings were "inexcusable" and that forces' good work risked being undermined if they failed to improve.
He said: "This is the third time we've looked at stop and search in the last three years and although there is some improvement, it's not happening fast enough.
"I am frustrated by the apparent lack of commitment by chief constables to ensuring stop and search is used properly and legitimately, and I am looking for police leaders to take action to address this within the next three months."
The Police and Criminal Evidence Act (PACE) requires officers to record each stop and search and include the grounds on which it was conducted, which must be reasonable.
The inspection looked at around 100 stop and search records in each of the 43 police forces in England and Wales.
An overwhelming majority of the 15% had grounds recorded which were not reasonable, with very few having no grounds recorded at all.
Inspectors did not find a single force where all stop and search records contained reasonable grounds.
Figures revealed a huge disparity across the police - in Cleveland Police force almost two-thirds of the records assessed did not have reasonable grounds recorded, compared to Nottinghamshire police where all but three records were complete.
Only 11 out of the 43 forces were fully complying with guidance in a scheme launched by the Home Secretary Theresa May, despite all being signed up to it, and 13 were not complying with at least three of the scheme's five requirements.
The report recommended chief constables carry out reviews at least twice a year to ensure justifications for stop and searches were accurately recorded.
Mr Otter said: "If you don't have reasonable grounds then the stop and search may be unlawful.
"It doesn't mean they are unlawful, it indicates they might be.
"What it doesn't show is that they were lawful."
For some black, Asian and minority ethnic (BAME) people, stop and search was "a symbol of their perception that there is a culture of unlawful discrimination within the police", the report acknowledged.
Mr Otter said: "Every single major report into disorder in this country since 1970 places stop and search as one of, if not the most important contributing factor, and those lessons need to be learned."
A lack of understanding and confidence about what reasonable grounds were was cited as a possible explanation.
He suggested that the decline in stop and search since 2010 could be due to police reacting to public perceptions that the tactic was used in a discriminatory way.
He said: "It does appear that this is a reaction to a concern about stop and search rather than a genuinely thought-through plan to better use the power."
The watchdog also highlighted that "the low proportion of stop and search encounters which result in an item being found does raise some concerns that the powers are not being used as effectively as they could be".
Mr Otter added there was no evidence to show that the rise in knife crime was linked to stop and search.
Overall, the report was promising. Some 37 out of the 43 police forces were rated as "good", while Kent Police achieved an "outstanding" grade.
Only five forces were rated "requires improvement" and none were branded "inadequate".
Police were applauded for using tasers fairly and appropriately in the first investigation to date into how police deploy the electric weapon.
Home Secretary Theresa May said: "Stop and search must be applied fairly, transparently and in a way which builds community confidence. That is why I introduced the Best Use of Stop and Search scheme.
"Forces who are signed up to that scheme must deliver on their commitments. It is unacceptable that 13 forces have been identified by HMIC as failing to comply with three or more requirements, and I have suspended them from the scheme with immediate effect.
"Those forces must now take this opportunity to improve their performance. Intelligence-led stop and search is an important police tool. But where it is misused it is unfair on the public and wastes police time."