More than a third of asylum seekers detained while their claims are being assessed are held for longer than three months before being removed, inspectors have said.

The detained fast-track (DFT) scheme, used when officials deem that an individual's circumstances are uncomplicated and a decision can be made quickly while they are held in detention, was not working as quickly as intended, a review found.

John Vine, the independent Chief Inspector of the UK Border Agency (UKBA), said it took longer than three months to remove those who had no right to stay in the UK in 38% of the 114 cases reviewed.

On average, decisions alone were taking 13 days to be made, despite the troubled agency's stated target of just three to four days. These delays in interviewing the asylum seekers led to more than £100,000 in extra costs in the sample cases alone, he said.

Mr Vine, whose reviews of the UKBA's work has led to Home Secretary Theresa May splitting off the work of the Border Force into a separate agency from next month, said he was concerned that despite the cost and impact of the DFT, there was no analysis of the scheme.

It was first brought in eight years ago when asylum applications were at record levels and now makes a "sizeable contribution to the overall number of people removed" from the UK, Mr Vine said. But the screening process was not tailored to determine whether an asylum seeker was suitable for the DFT system.

"While safeguards were in place once people had been detained, there remained a particular risk that the victims of torture or trafficking could be allocated to the DFT contrary to the agency's own policy," he said.

Mr Vine added: "I found that the DFT was not working as quickly as intended. People waited on average 13 days in detention before a decision was made on their claim, compared with the three to four day timescale the agency needed to meet - this is a significant disparity. The length of time between interview and decision also has cost implications for the taxpayer."

A UKBA spokesperson said: "The DFT system plays a fundamental role in making sure asylum claims can be processed and decided as swiftly as possible. This helps those who are granted asylum quickly build lives within the country, while ensuring we can remove people who have no right to remain in the UK.

"Around three quarters of people whose claims were refused were removed from the UK. Of those almost two thirds were removed within three months. However, we are allocating more staff to help speed up the processing of these cases."