Tony Blair accused the Conservatives of seeking deliberately to exploit people's fears over immigration and asylum today, saying that the Tories had "gone from being a one-nation party to being a one-issue party".

In what was probably his most important speech of the election campaign so far, the Prime Minister sought to regain ground lost to the Conservatives over the issue by promising to detain and deport more failed asylum seekers and use electronic tagging to keep tabs on others.

He also confirmed plans announced earlier in the day by Charles Clarke, the Home Secretary, to recruit 600 more border guards.

Making the speech in Dover, where the immigration issue is especially sensitive, Mr Blair spent much of his time attacking the Conservatives' promise to fix an upper limit on immigration and process all asylum claims "in a fantasy offshore asylum-processing centre".

He said of the Conservatives: "Their campaign is based on the statement that it isn’t racist to talk about immigration. I know of no senior politician who has ever said it was.

"So why do they put it like that? It is an attempt deliberately to exploit people’s fears, to suggest that for reasons of political correctness, those in power don’t dare deal with the issue - so that the public is left with the impression that they are being silenced in their concerns, that we are blindly ignoring them or telling them that to raise the issue is racist, when actually the opposite is true."

Mr Blair defended his Government's record on tackling the issue of asylum and illegal immigration, saying that asylum applications had dropped from a peak of around 8,000 per month in late 2002 to a little more than 2,000 per month now.

The time taken to process applications had also dropped sharply because of streamlined procedures and more and more failed asylum-seekers were being deported, he said.

Mr Blair said a third-term Labour government would restrict immigration through an Australian-style points system, matching those who come to the UK with skills needed in the economy. He also confirmed that Labour would reintroduce legislation for biometric ID cards as soon as possible if they are re-elected.

He said: "On asylum, we want fast-track processing and removal of as many unfounded applicants as possible with more detention and the use of electronic tagging where there is a risk of asylum applicants disappearing.

"We have set a target of removals exceeding applications for the first time ever. To speed up removal of those who lose or destroy their documents before claiming, we will introduce finger-printing of all visa applicants before they travel to the UK to prevent people concealing their identity after entry and we will ask airlines to copy documents before people board on more high risk routes."

Mr Blair said that a Conservative proposal to pull Britain out of the UN's Refugee Convention, that obliges it not to turn away genuine refugees, would leave the country in international isolation, unable to work with partners such as the UNHCR refugee agency to manage the problem.

It was Labour's most direct attack on the Conservatives over the only issue which, according to pollsters, they enjoy a clear lead with voters - even though recent polls suggest that Mr Howard's hard line on immigration might also be costing the Conservatives votes.

"Concern over asylum and immigration is not about racism. It is about fairness," he added.

"But I never want this to be an issue that divides our country that sets communities against each other. We are a tolerant, decent nation. That tolerance should not be abused. But neither should it be turned on its head.

"It is the duty of government to deal with the issues of both asylum and immigration. But they should not be exploited by a politics that in desperation seeks refuge in them."

David Davis, the Shadow Home Secretary, earlier defended his party’s stance, describing it as an "entirely responsible and entirely sensible" attempt to head of a long-term problem.

"Every responsible government in this country over the years has recognised the need for controls on immigration," he said. "This Government, interestingly, seems to think there’s no upper limit. We disagree."

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