Liberty is being kept safe from terror, says Brown as Davis warns of continuing threat to civil liberties

In a speech on 'liberty and security', the Prime Minister defended his decision to push ahead with the 42-day detention of terror suspects without charge, but stressed the need to balance protection with long-held British freedoms.

Addressing the Labour-supporting think tank IPPR, he said that 'the first liberty is to be free from fear' and stressed that he would not back off tougher measures to combat crime and terrorism.

Gordon Brown has defended his decision to push ahead with the 42-day detention of terror suspects despite staunch opposition from Tory MP David Davis (right)

He added that criminals and groups such as al Qaeda were using '21st century methods' and governments must use '21st-century methods'  in response.

That meant allowing police the time needed to decode complex computer messages used by plotters against the state.

Mr Brown said that at every stage of his plans, from 42-day detentions to the use of CCTV and DNA databases, safeguards to protect civil liberties had been introduced.

His speech came as a poll showed that Mr Davis had won public support for his decision to fight a by-election over the 42-day issue.

The nationwide IpsosMORI survey for the Independent found that 35 per cent would vote for the former shadow home secretary if they lived in his Haltemprice and Howden constituency.

Some 23 per cent said they would not support him, while 33 per cent said they would not vote and nine per cent replied 'don't know'.

But the poll, the first national survey on the issue since Mr Davis announced his decision last Thursday, also found that voters were split over whether he was right or wrong to resign.

While 39 per cent believed he was right, 48 per cent said he was wrong, and 13 per cent replied 'don't know'.

Shadow chancellor George Osborne revealed today that he found out about Mr Davis's decision only minutes before Liberal Democrat leader Nick Clegg was informed.

He admitted he had been shocked by the move but praised his colleague's 'courage and bravery'.

'He is fighting a crusade on this issue and I wish him well with that. He's an extremely principled man,' Mr Osborne told GMTV.

Mr Davis is set to formally resign tomorrow after Prime Minister's Questions, by taking up the Chiltern Hundreds, a convention designed to allow an MP to vacate his seat.

He will not be making a speech in the Commons.

IpsosMORI found that men (51 per cent) are more likely to think that Mr Davis was wrong than women (44 per cent).

A majority (51 per cent) of 18-24-year-olds believed he was right to resign, with 31 per cent saying he was wrong.

The figures were reversed among older voters. Among those aged 65 and above, 54 per cent believed he was wrong and 32 per cent right.

Mr Davis said: 'This debate has only just begun. But already, we are seeing declining support for 42 days, as the arguments are aired and the public engage on this vital national issue.'