The scrapping of control orders for terror suspects moved a step closer last night when the security services appeared to dramatically soften their support for the controversial measures.

In a surprise development, senior Whitehall security sources broke ranks to reveal that MI5 was "not wedded" to keeping the contentious regime.

While conceding that control orders, which curtail suspects' freedoms, had proved useful in the past, sources stressed that this was quite different from saying the security services were demanding the orders be retained.

Ministers are reviewing counter-terrorism laws and are due to report next month. The home secretary, Theresa May, was thought to have been convinced of the need to retain the orders as a "necessary evil" in the fight against terrorism.

Only six weeks ago a high-profile speech by the director-general of MI5, Jonathan Evans, appeared to call for the government to maintain the policy of putting terror suspects under house arrest without trial .

The signs that MI5's previously unequivocal backing for control orders has receded has raised hopes among civil liberties campaigners that one of the most divisive aspects of anti-terror policy might be abandoned.

Shami Chakrabarti, director of Liberty, said: "If it is right that the security service is rowing back on control orders, this destroys any possible argument for ministers breaching the promises of opposition and the commitment to liberty that was supposed to bind the coalition together.

"It also creates – after a difficult period – the possibility for a new, more positive dialogue between security and legal communities around the shared objective of dealing with terror within the rule of law," she added.

The issue of control orders has already caused deep divisions within the coalition. Former shadow home secretary David Davis warned last week that the government was heading for a "problem on a major scale" if it failed to get rid of control orders.

The Liberal Democrats pledged to scrap them during the general election. Lib Dem cabinet minister Chris Huhne said last week that keeping control orders would undermine key British values.

May has clashed with Lord Macdonald, a former director of public prosecutions and the Lib Dem peer appointed by her to lead the counter-terrorism review. Macdonald is believed to have warned May he will publicly condemn any decision to retain the orders when the review is finally published. May responded last week by attempting to downgrade his role in the review.

Senior Whitehall sources said last night that David Cameron was yet to make a final decision on control orders, but said that the security services understood it was a "purely political decision".

The Observer has learnt that the Home Office has extended the contract of Lord Carlile, the government's adviser on counter-terrorism laws and one of the most high-profile advocates of control orders. A report by the Lib Dem peer last February warned that abandoning the system could damage UK security. He said there was "no better means of dealing with the serious and continuing risk posed by some individuals" than control orders. His contract should have expired in September but Home Office sources have confirmed that his contract - parliament has not even been informed - has been extended until the end of the year.

The extension means that Carlile will be employed by the coalition when May's review of counter-terrorism laws is published. Chakrabarti said: "Lord Carlile is about as neutral on these matters as I am and that makes him ill-suited to be reviewer of anti-terror laws.

"This statutory appointment is crying out for a judicial figure to provide detailed reports on the operation of controversial measures; not be a public cheerleader for internment."

Some senior Conservative ministers are also opposed to keeping the regime, which has been recently been undermined by a series of key court rulings.

Nine British citizens suspected by the government of involvement in terrorist activity are currently being held under the system, part of counter-terrorism legislation introduced in 2005.