Plans to further increase the time for which terrorist suspects can be held without charge have been criticised by England's top prosecutor.
Sir Ken said 28-day detention was working
Sir Ken Macdonald, the Director of Public Prosecutions, said extending the 28-day limit to 42 days was legislating "on the basis of hypotheticals".
Sir Ken said the current time limits were working for prosecutors.
The government argues that the increasing complexity of cases requires an increase in the limit.
Prime Minister Gordon Brown is expected to face significant parliamentary opposition to his plans to extend the limit.
Sir Ken told BBC Radio 4's The World At One programme: "The basic point is whether you want to legislate on the basis of hypotheticals or whether you want to legislate on the basis of the evidence that we have acquired through practice.
"It seems to me that if you are legislating in an area which is going to curtail civil liberties to a significant extent, it is better to proceed by way of the evidence and the evidence of experience."
He added: "We have found that a period of 28 days does work for us as prosecutors.
"We can charge on the basis of a 'reasonable suspicion' within that period so long as we anticipate that sufficient evidence to provide a realistic prospect of conviction will be forthcoming within a reasonable period."
Sir Ken said if it had not been possible to get enough evidence to provide reasonable suspicion in 28 days, then it would be hard to convince a judge that it could be achieved in 35 or 42 days.
Former Prime Minister Tony Blair suffered his first Commons defeat when he tried to extend the limit to 90 days.