Tens of thousands of illegal migrants were allowed to stay in the country after telling police that they were British or EU citizens, a report has found. In response, MPs have accused the police of “taking their eye off the ball” on immigration.

The report, by the Independent Chief Inspector of Borders and Immigration found that in some areas across the country, police officers were taking migrants at their word regarding their right to remain in the country, rather than referring them to the Home Office for checks on their eligibility to stay in the UK as protocol demands.

According to police figures, between 185,000 and 193,000 foreign nationals were arrested by police forces in the UK between 1 April 2014 and 31 March 2015, of whom about a third were arrested by the Metropolitan Police Force, which covers London.

In 2012 police procedures on the arrest of foreigners were changed. Previously officers could phone the Home Office for an immigration status check on those detained, but since that date officers have been expected to refer the detained person to the Home Office Command and Control Unit (CCU) or an embedded Immigration Officer (IO) for processing.

This does not appear to be happening in a significant number of cases.

“[The] inspection found that not all foreign nationals arrested in the UK by the police were referred to the Home Office for an immigration status check. It was not possible to say how many arrested foreign nationals went unchecked,” the report noted, adding: “Home Office data indicated that the level of referrals varied across the UK.”

Whereas Met officers were believed to be referring all or nearly all foreign detainees to the CCU, officers with the West Midlands were found not to be referring a significant number of people.

The report found: “When embedded IOs were able to check lists of people in custody, they routinely found that 5-10 percent of the foreign nationals detained had not been referred. Both EEA nationals and those self-declaring as UK citizens were likely to be missed.

“IOs were reliant on police custody sergeants to give them access to the custody database to ensure that all foreign nationals were identified and referred for a status check, but when the custody suite was busy this was not always possible.”

Officers claimed in some cases not to be aware of the need to refer people to the CCU or “might assume that the offender was unlikely to have committed an actionable immigration offence, so would not see any advantage in involving immigration officers,” the report found.

But in other cases, migrants were simply able to claim that they had a right to stay – and were believed.

“Not all foreign nationals arrested by the police are identified as such,” the report states. “Some misrepresent themselves as UK citizens or as nationals of a country within the European Economic Area (EEA), with the hope of evading immigration checks. Meanwhile, police are less likely to refer genuine EEA nationals for an immigration check, assuming that they are free from immigration control in the UK.

The report did not look at the effectiveness of any enforcement action taken by the Home Office after a foreign national had been referred by the police, meaning that many more who were referred for action may have escaped deportation nonetheless.

A Home Office spokesman said: “We are identifying and removing more foreign criminals from the UK than ever before.

“As the Home Secretary recently announced, we are also introducing stronger powers to deport EEA criminals and stop them returning to the UK.”

Conservative MP Tim Loughton, acting chairman of the Home Affairs select committee, said: “At a time when the government is focused on reduced immigration numbers some police forces appear to have taken their eye off the ball when dealing with foreign nationals in the UK who may be committing offences.

“It is all very well for Government to say they are deporting more foreign national criminals than ever but they are starting from a very low base.”