The Government continued to lobby Labour MPs and put pressure on the Conservatives today to support its proposal for a 90-day detention period for terror suspects.

In the last 24 hours before the proposal comes before the House of Commons, Charles Clarke, the Home Secretary, and Gordon Brown, the Chancellor, both campaigned strongly for the most controversial element of the Terrorism Bill.

Sir Ian Blair, the Commissioner of the Metropolitan Police, also held a previously-scheduled briefing for journalists in Westminster this afternoon, in which he offered his unqualified support for the 90-day limit.

Sir Ian acknowledged objections to the proposal but said that police were facing an entirely new threat. "I have been in the police service 30 years, I have been a top cop for 10 years," he said.

"I have never seen anything like what is happening at the moment. There are people out there in this country plotting mass atrocities without warning. Under these circumstances we do believe that the state has a duty on behalf of its citizens to give the greatest level of protection it possibly can. This is different. It is chilling, what we are seeing, and we are very, very worried and alarmed about it."

Sir Ian said the 90-day proposal, which the Government has gradually insisted upon despite sustained resistance from the Conservatives, Liberal Democrats and a group of Labour backbenchers, was the best estimate of the time police needed to mount complex anti-terrorism investigations.

"There is no magic in 90 days, we are not saying it is 90 rather than 89, but in 90 versus 60 or 90 versus 30, we are at 90," said Sir Ian. "We are sticking at 90 because our experience, in a number of trials, a number of investigations, is this is the length of time it is taking to actually get a coherent picture of what we have in front of us."

The advice of senior police officers was the basis of the Chancellor's rallying call for the measure. Mr Brown broke his long silence on the Terrorism Bill, elements of which came within one vote of defeat last week in the House of Commons, to express his support for the police and attack Tory opposition to the plan.

"When you have advice from the Association of Chief Police Officers, the head of the Metropolitan Police and those specialists concerned in the detailed work of interviewing and dealing with terrorist suspects, and they are all saying the same thing about the need for the extra time, then you ought to, and will, take that very seriously indeed, as we have done," he said.

Mr Brown then criticised the politicking of the Opposition: "I am surprised and shocked at the short-term opportunism of our opponents in the Conservative Party, particularly the two leadership candidates in the party who wish to put short-term political opportunism before the long-term strategic interest of our country."

Earlier, the Home Secretary followed the fighting lead of Tony Blair, who spent yesterday vigorously campaigning, first before the press and then a meeting of Labour MPs, for the 90-day rule. Mr Clarke told MPs planning to vote against the measure to "face reality".

"People have to face up to an issue which exists now, which is of a terrorist threat to this country which is real and substantial, as tragically we saw on 7 July," the Home Secretary told BBC Radio 4's Today programme.

"My criticism of some others is they address the environment that is around but don’t face up to that absolutely core central reality, which is a reality of modern life," he said.

"The reason why I think the people of the country, as manifested in poll after poll, support what the police are proposing in these areas is they see that reality, they understand it, and some others don’t."

The Government has been buoyed by the warm reception of its promise to add a "sunset clause" to the 90-day rule, under which the proposal would lapse after 12 months and have to be re-endorsed by Parliament. A Populus poll for The Times also showed overwhelming public support for the 90-day option.

Despite the optimism of Mr Blair - his official spokesman said there today that there was "a growing acceptance" of need to hold terrorist suspects for three months before charging them with a crime - the Conservatives Party maintained its opposition to the plan this evening.

In an interview on BBC Radio, Michael Howard said his party was sticking to its limit of 28 days and suggested that police advice on the holding of suspects should be subjected more scrutiny.

"There has been no justification whatever of 90 days. We don’t believe 90 days can be justified," said the Tory leader. "If you actually look at what the police are saying, the logic of what they are saying is that there shouldn’t be any limit at all and that we should have quite unlimited detention without charge."

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