Almost half the asylum cases dealt with by the Home Office are being classified as “non-straightforward” excluding them from a six-month decision target, an official watchdog has revealed.

The chief inspector of borders and immigration says that an internal Home Office review earlier this year revealed that a significant number of these claims “should never have been categorised as non-straightforward”.

David Bolt’s report on the processing of asylum claims in Britain shows lengthening backlogs in dealing with asylum cases despite a 20% reduction in the number of claims to 29,549 in the 12 months to March 2017.

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The report published on Tuesday reveals that more than half of those classified “non-straightforward” had faced delay of more than 12 months in getting a decision as of March 2017 and fewer than a quarter had had an initial review.

It also discloses that more than a quarter of Home Office staff who take decisions on asylum cases quit over a six-month period. The asylum casework inspection also reveals that when a special project based in Bootle was set up to clear the backlog of “non-straightforward” cases only one member of staff had been recruited by the time it was supposed to start in July this year.

The chief inspector of borders said the Home Office had described the asylum system as “in transition” and there were plans to enable it to cope better with peaks in demand.

“My message is the Home Office needs to accelerate these plans and put itself in effective control of the asylum process as soon as possible. Otherwise, the next peak in asylum intake, or trough in staffing levels, will see it fall further behind.”

In particular Bolt singles out the treatment of pregnant asylum seekers whose claims are one of seven case types that are categorised as “non-straightforward”. He said although their numbers were relatively low they appeared to be disadvantaged.

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On 31 March there were 121 asylum claims categorised as “non-straightforward” because they had been made by pregnant women. Of these, 67 had been waiting more than a year for a decision, while 17 had not had an initial decision more than two years after claiming asylum.

The chief inspector says that of the 20,385 cases waiting for an initial decision in March, 10,637 or 52% were classed as “straightforward” and nearly all being dealt with in the six-month customer service target. But the claims of the remaining 48% or 9,748 asylum seekers had been classified as “non-straightforward” – 2,434 more than a year earlier when they formed only 39% of the “work in progress”.

Bolt says that that the challenge facing the Home Offices asylum intake and casework unit was not made easier by a large turnover in staff, lengthy staffing gaps, and by the inexperience of the new staff who were recruited.

The number of asylum “decision-makers” fell from 319 in January 2016 to 228 in July 2016 – with a quarter leaving in the space of six months – but staffing numbers recovered to 352 by March this year.

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However their effectiveness was hit by a decision to divert asylum decision-makers to deal with the arrival of children from France when the Calais camp was cleared and by the inexperience of new staff. Internal Home Office data found 25% of decisions were unsatisfactory.

A Home Office spokesperson said the report recognises the commitment of our staff who worked in the challenging and high-pressured area of asylum casework.

“However, we are not complacent and we accept the independent chief inspector’s conclusions that some changes are needed.”