The asylum system is in turmoil and claimants could now be offered a backdoor amnesty to remain in Britain, a leaked Home Office memo has revealed.

The document raises fears that a government target to speed up the processing of new claims could lead to existing cases being given "lower priority", potentially allowing thousands of claimants to stay in the country indefinitely.

The memo, seen by this newspaper, says unrest is spreading in detention centres, a growing number of claimants are going missing before cases are decided and the number of failed asylum seekers being deported is declining.

Damian Green, the Conservative immigration spokesman, said: "This leak lays bare the chaos that still affects the asylum system. Ministers have toughened up their rhetoric, but underneath the same old policies are producing the same old results.

"Foreign prisoners put into immigration detention centres are causing continuing unrest. We have already seen one detention centre partly burnt down, and riots in another. The response of the Government is to declare a stealth amnesty."

The Home Office has been set a "conclusion rate" target that 40 per cent of next year's asylum claims should be decided within six months of being lodged, rising to 90 per cent by 2011; and, officially, ministers have ruled out a formal amnesty for asylum seekers or illegal immigrants, and insist that decisions will continue to be taken on a case by case basis.

But, in the leaked memo, headed "Fast track asylum decisions - business performance", officials state: "There is a risk that in order to achieve the conclusion rate, [the asylum system] will increasingly focus on cases that can meet the target. When any case becomes over six months old, it may receive lower priority than those cases that have a chance of meeting the target."

The result, the memo warns, could be a build-up of older cases which would clog up the system. Staff will be asked to think up "quick win" improvements to tackle the problem.

In a damning assessment of performance in the period up the start of August, the memo states: "Intake in recent weeks has been higher than the same time last year, reversing the downward trend on asylum intake. Levels of absconding appear to be increasing."

Officials also raise concerns that a separate exercise to clear 450,000 "legacy" asylum cases, some dating back a decade, will be perceived by the public as a full-scale amnesty.

John Reid, the former home secretary, announced last year that the legacy backlog would be cleared by July 2011. A special team was set up to process the cases. Sources say that most claimants will be allowed to stay on the grounds that they are already settled, which the memo warns "may be perceived as an amnesty".

Home Office figures show that only 23,610 people claimed asylum last year, the lowest annual total since 1993. However, the Government missed its deportation target with only 6,780 failed asylum seekers removed in the first half of this year, down from 10,345 in the same period last year.

With fewer than 3,000 places in immigration detention centres, most asylum seekers are given free housing while their claims are processed, making it easy for them to abscond.

Since last year's crisis, when it emerged that foreign prisoners had been released without being considered for deportation, many detention places have been occupied by former prisoners, adding to the unrest in the centres.

There was rioting and arson at Harmondsworth detention centre, near Heathrow, last November, while in March seven staff and two inmates were hurt in a riot at Campsfield House, Oxfordshire.

Countries across western Europe have seen a decline in asylum numbers in recent years. Tough new security at Channel ports has helped to prevent migrants from entering Britain as stowaways, although there are still daily attempts to breach security at ports such as Cherbourg