Legislation raising the prison term for anthrax hoaxers to seven years will begin from Sunday following the announcement of the new laws by the Home Office.

When the government introduces its new anti-terror legislation in November it will be backdated to Sunday so that anyone found guilty of hoaxes over germ warfare can be sentenced to up to the full seven-year term.

The new laws follow hundreds of hoax calls and packages that were sent around the country following the wave of anthrax letters sent to politicians and media organisations in the US. Tony Blair and home secretary David Blunkett agreed the move at the latest meeting of the war cabinet on Thursday

The prime minister's official spokesman described the measure as "an exceptional measure but these are exceptional times".Liberal Democrat home affairs spokesman Simon Hughes claimed backdating served to underline the government's distain for the House.

"The government must learn that it's the role of ministers to propose legislation and the role of Parliament to decide whether that legislation will be passed."

John Wadham, director of civil rights group Liberty, said backdating the law breached an important legal principle.

"What we need from government is careful, well-considered measures ... not rushed, ill-thought measures that cut across the basic principles of democratic government and the basic rule of law.

"It's a traditional principle of British law, and of our constitution that you don't make retrospective law so you can punish people more severely for offences they've already committed," he said.