Gordon Brown defended his government's controversial security legislation on Tuesday, saying the country cannot have its "head in the sand" in its fight against terrorism.

As its citizens became more fearful of another attack, Britain needed "21st century methods to deal with 21st century challenges", the Prime Minister said in a keynote speech.

Britain cannot have a "head-in-the-sand approach that ignores the fact that the world has changed with the advent of terrorism", he said in the speech to the Institute for Public Policy Research.

His speech came days after Conservative law and order spokesman David Davis quit parliament to force a mid-term vote for his seat, vowing to stand again to fight the "strangulation of fundamental British freedoms by this government".

The balance between civil liberties and security policy is shaping up to be a major political issue in Britain.

The Conservatives say a bill that barely passed parliament to increase the time terrorism suspects can be held without charge to 42 days is a threat to civil liberties.

Downing Street denied the prime minister's speech was a response to the resignation of Davis, which Brown has called a "stunt that has become a farce". The Labour Party has not said if it will field a candidate against Davis.

In his speech, Brown said policing methods such as closed-circuit television and nationwide DNA databases had helped to cut crime significantly. New laws designed to fight terrorism, including the 42-day pre-charge detention policy, actually improve civil liberties not hamper them, he said.

The laws were necessary to protect British citizens against "2000 known terrorist suspects, 200 organised networks and 30 current plots," Brown said.

"Just as we need to employ these modern means to protect people from new threats, we must at the same time do more to guarantee our liberties."