Control orders "haven't been a success" and need a "proper replacement", Prime Minister David Cameron has said.

The future of the anti-terror measure has been a cause of contention between Tories and Lib Dems, but Mr Cameron said he was "confident" of agreement.

The Lib Dems promised in their election manifesto to replace control orders, but some Tory MPs want to keep them.

Deputy Prime Minister Nick Clegg said the coalition government would make an announcement "very soon".

Introduced under the former Labour government in 2005, control orders allow ministers to place a terrorism suspect under close supervision that some say is similar to house arrest.

Opponents say this infringes civil liberties, but supporters argue it is necessary to protect the public.

'Freedoms'

A review of control orders was announced in July and is being overseen by former director of public prosecutions and Lib Dem peer Lord Macdonald.

The results had been due to be published before Christmas but this was delayed amid reports of policy disagreements between the coalition government partners.

Mr Cameron, on a visit to Leicester, told the BBC: "I think we need a system that keeps the country safe but that respects our freedoms. Nick Clegg and I are working very hard to bring this about."

He added: "The control order system is imperfect. Everybody knows that. There have been people who've absconded from control orders. It hasn't been a success. We need a proper replacement and I'm confident we'll agree one."

Mr Cameron also said: "It's not about a victory for the Conservatives or the Liberal Democrats. It's about trying to do the right thing for our country, for the security of our country and our civil liberties."

The BBC understands that discussions have reached detailed negotiation about the powers that should be put in place instead of control orders. They include curfews, relocation requirements, electronic tagging, travel bans, restrictions on the use of telephones and computers and on who the suspect can meet and where they can go.

It's about time the coalition honoured past promises to dump this Blairite legacy and concentrated on investigating and prosecuting terrorists Isabella Sankey, Liberty

BBC home affairs correspondent Danny Shaw said there are currently only eight control orders in force and a few of those were believed to involve "very light touch" measures.

Earlier, while canvassing in the Oldham and Saddleworth by-election, Mr Clegg said counter terrorism measures were being reviewed because "everybody in the coalition, Conservative and Liberal Democrat, thinks the last government didn't get the right balance between liberty and security.

"Of course our absolute top priority must always remain the safety of the British people but we mustn't do that by sacrificing traditional British liberties which have always made this country very great."

He added: "We're going to make an announcement very soon I hope but it's a very important task this, it's a painstaking task to get the balance between liberty and security right and that is what I'm committed to do."

But Isabella Sankey, of campaign group Liberty, said: "Describing control orders as 'imperfect' is like calling the Titanic 'not quite afloat'. Under this mad scheme some terror suspects face indefinite punishment without trial whilst 15% have completely disappeared.

"It's about time the coalition honoured past promises to dump this Blairite legacy and concentrated on investigating and prosecuting terrorists."

The previous Labour government argued it had to control some suspects who could not be prosecuted because secret intelligence was not allowed in British trials.

But shadow home secretary Ed Balls indicated last autumn that, if police and security services could persuade the home secretary that alternatives such as travel restrictions and increased surveillance could work, Labour should support a change.

Former Conservative leader Lord Howard said this week that control orders had to stay because there was "no alternative way" of preventing terrorist attacks in the UK.

And the government's independent reviewer of anti-terror laws, Lord Carlile, agreed, saying the police and security services had made a "clear case" for their retention.

The review of control orders is also looking at the 28-day limit on holding terrorism suspects before they are charged.