Home secretary Charles Clarke is under pressure to explain whether a foreign terror suspect was allowed to stay in the UK after release from jail.
Home Secretary Charles Clarke has faced mounting pressure
If true, this would be "an unforgivable failure to protect the public against the most serious threat", shadow home secretary David Davis said.
The man served a prison term for robbery and was released in 1998, but there are no records about deportation.
The Liberal Democrats' Nick Clegg called for urgent clarification.
The Home Office said it only started keeping detailed records of deportation in 1999, so had no records of whether the terror suspect was recommended for deportation.
The story first emerged in a blog by political editor Martin Bright in the New Statesman.
The terror suspect arrived in Britain in the early 1990s as an asylum seeker and was given exceptional leave to remain. He was convicted in 1996 of taking part in a robbery and was freed two years later.
A spokesman for the department said it was unlikely a recommendation on deportation would have been made as he was under 18 at the time of sentencing.
The probation officers' union Napo said risk assessment of prisoners was "not an exact science".
Assistant general secretary Harry Fletcher said: "It would not have been possible to predict that lesser offences in this case could later lead to someone being accused of involvement in terrorism."
The latest case comes after calls from opposition parties for Mr Clarke to resign because his department allowed 1,023 foreign prisoners to be released, over seven years, without being considered for deportation.