The director of public prosecutions has told MPs he will not be pressing the government for the current 28 day limit for holding suspects to be extended.
Sir Ken MacDonald said prosecutors had "managed comfortably" with 28 day detention and extending it would be based on "hypothetical" evidence.
Ex-Attorney General Lord Goldsmith also appeared before the home affairs committee to cast doubt on the plans.
Minsters and police have said they want to extend detention without charge.
They say it is necessary because of the increasing complexity and international nature of terror plots being investigated by police.
But Conservative, Lib Dem and some Labour MPs remain to be convinced, setting up the possibility of a Commons defeat for the government if it decides to put it to the vote.
Sir Ken told the MPs: "There are reasonable arguments for detention and I respect those arguments.
"Our experience has been that 28 days has suited us quite nicely."
He said it was up to MPs whether they extended detention beyond 28 days but it would be based on "hypothetical" cases.
"We have had very, very complex cases since this law was enacted and in only three have we had to go beyond the 14 days.
"Of course, evidence is increasingly complex and that's why we have gone up to 28 days, which is by the far the greatest period in the common law world."
He added: "Our experience so far has been that we have managed - and managed reasonably comfortably.
"Of course it's always possible to set up hypothetical situations in which it could become extremely challenging - and it's for Parliament to decide whether it wants to proceed on the basis of hypotheticals - rather than the evidence we have received so far," Sir Ken told MPs.
Sir Ken had asked for more time for police to question suspects when the limit was 14 days - but he would not be asking the government to extend it beyond 28 days.
He also suggested prosecutors may have difficulty convincing a judge to extend detention, even if the legal maximum was increased.
"If after 25 or 26 days you couldn't find a reasonable suspicion to justify a charging decision it might be quite difficult for a prosecutor to persuade a court," said Sir Ken.
Former Attorney General Lord Goldsmith, giving evidence to the same committee, said he was "not persuaded" of the need to extend detention beyond 28 days.
And he said he would have resigned had Tony Blair's government succeeded with their attempts at 90 day detention.
He told MPs Mr Blair was "aware of my concerns" but he had not wanted to put the then PM "over a barrel".
Lord Goldsmith, who quit as attorney general when Gordon Brown took over as prime minister, said: "If the 90 day proposal had come from the Commons unamended, I would not have found it possible to vote for it."
He said his view was based on the evidence he had seen when he was in government and he accepted the situation could have changed.
"I didn't see any evidence during my time to indicate that longer than 28 days was necessary," he told MPs.
That included one case in which questioning reportedly went "right up to the wire," said Lord Goldsmith.
He said any figure on detention was "arbitrary" but he thought 28 days was "right".
Lord Goldsmith said post-charge questioning, which is also being considered, could remove further the need for an extension.
But he also warned against "browbeating" suspects and "continually questioning them when there isn't any new material at all".
Giving his reaction, shadow home secretary David Davis said: "There can be no starker demonstration of the importance of this issue that the last Attorney General would have resigned over a central plank of the prime minister's policy.
"Neither the Director of Public Prosecutions, nor the last Attorney General, have seen the evidence to go beyond 28 days.
"Terrorism will be defeated by good intelligence, professional policing and the rigorous application of British justice, not by unnecessary incursions into the freedoms and rights that British subjects have had for centuries."
Tony Blair suffered his first Commons defeat as prime minister in 2005 when MPs voted to reject 90 day detention.