Holding terror suspects for up to 28 days will not significantly enhance police powers, the independent reviewer of anti-terror laws says.
Police had recommended 90-day detention without charge
Liberal Democrat peer Lord Carlile said he felt police needed 90 days to investigate possible offences.
That proposal has been rejected by MPs, who voted in favour of a 28-day limit.
Lord Carlile said he knew of "at least two or three" cases where suspects could not be prosecuted on appropriate charges because police ran out of time.
If the Terrorism Bill currently going through Parliament did not redress that situation, it would not be good enough to deal with the problem, he warned.
Currently terror suspects must be charged or released after 14 days.
Police have complained this does not give them enough time to plough through reams of information stored on computers, watch hundreds of hours of CCTV film and follow up leads often spanning several foreign countries.
"I don't myself think that increasing it to 28 days is going to make a very significant amount of difference." Lord Carlile told BBC Radio 4's World This Weekend programme.
"This whole debate has been about an extremely small number of cases - maybe one or two a year - but cases of the utmost importance.
"I believe I know of at least two or three cases in which a longer period of detention would have enabled the right people to be charged with and convicted of the right offences.
"If we don't introduce law that enables that to happen then we are not introducing law of sufficient quality."
However he said he understood MPs complaining that they had not been persuaded by the evidence that 90 days was appropriate.
"The problem is that a lot of evidence in the area of national security cannot be published."
As the independent reviewer, Lord Carlile can see unpublished information.
"I have absolutely no doubt that the prime minister was accurate in describing the potential danger caused by al-Qaeda terrorism.
"My assessment therefore was completely evidence based," he added.
The rejection of the 90-day plan last week prompted Prime Minister Tony Blair to say he hoped MPs did not "rue the dayż they made that decision.
MPs voted against by 322 votes to 291, with 49 Labour MPs rebelling.
Lord Carlile said it was "rather unsatisfactory" that the 28-day limit agreed upon by MPs did not feature the same weekly judicial oversight offered by Home Secretary Charles Clarke in the case of the 90-day option.
He said he lamented the way in which the terror debate had become entangled in what he called "big politics", adding it should be developed outside party politics.