The government should act now to secure the UK's borders and to seek new powers to deport asylum seekers judged a security risk, according to new Tory proposals.

The UK should also consider withdrawing from relevant international agreements if it proves the only route to a "fairer and more humane asylum system" said Tory leader Iain Duncan Smith.

The message to terrorists must be this: 'Stay at home or you will be sent home' Iain Duncan Smith

Shadow home secretary Oliver Letwin accepted the plans were "controversial" and would involve "significant" costs.

But he stressed they needed to be taken to address the current terrorist threat, which he described as a national emergency.

The plans include security checking asylum seekers at detention centres, which could be housed on ships, before they are admitted to UK towns or cities.

The Tories say new government ideas on asylum have come out of "panic" and argue terrorists must not be allowed to abuse the asylum system.

'Cavalier' attitude

Mr Letwin called on the government to provide full and effective 24-hour surveillance of ports of entry and to ensure all vehicles are checked for hidden people.

Unveiling the plans on Tuesday, Mr Letwin said: "It is open to those terrorists that enter either clandestinely or with visas to claim asylum and use the present chaos in the asylum system to disappear."

Asylum has previously proved a tricky tightrope for the Tories

Mr Letwin said ministers should enter a reservation against article 3 of the European Convention on Human Rights to give Britain the power to deport foreign nationals judged to be a risk to the public.

Article 3 of the European convention outlines the right to freedom from torture or inhuman or degrading treatment.

Tory leader Iain Duncan Smith warned: "When people present themselves for entry they should be admitted to our towns and cities only when we are confident they pose no security threat...

"We must also ensure that we have all the powers necessary to deport those who fail to get through that process.

"The message to terrorists must be this: 'Stay at home or you will be sent home'."

Cost worries

The Conservatives plan to look at asylum regimes in Australia and the Netherlands.

But Margaret Piper, from Australia's Refugee Council, said detention schemes there cost �100 per person per day.

"If you are talking about 60,000 or 70,000 people entering the country a year, I'm not entirely certain the British taxpayer would wear it", Ms Piper told BBC Radio 4's World At One.

Hughes: Labour and Tory positions "disgraceful"

The Tories are also looking at how to "renegotiate" the 1967 New York protocol to the 1951 Geneva Convention and Dublin Convention.

Mr Letwin said: "We must be prepared to withdraw from the relevant international agreements if it is the only way to create a fairer and more humane asylum system.

"We are very well aware that the proposals we have outlined are highly controversial, but we are not afraid of that controversy."

'Change under way'

Immigration Minister Beverley Hughes immediately condemned the plans as "ineffective, uncosted and hypocritical".

Ms Hughes said: "We are already fully engaged in the challenge of implementing a reformed asylum system."

Civil rights group Liberty also branded the proposals "illogical".

Liberty director John Wadham said: "Tens of thousands of innocent people would be detained in the vague hope it may turn up a tiny handful of suspects."

Downing Street denied claims Tony Blair tried to undermine the Tory initiative by suggesting on Sunday the UK might quit the European convention.

Number 10 said new measures, such as accommodation centres, tighter port security and benefits changes would first be given time to work.

EU withdrawal?

Liberal Democrat spokesman Simon Hughes said both government and Tory proposals were based on "fear and ignorance".

"Both the prime minister and Iain Duncan Smith now seem to believe that it is a matter for regret that we cannot send people back to countries where they would be tortured," said Mr Hughes.

"That is a disgraceful and unprecedented position for the leaders of major political parties to take."

Guy Goodwin-Gill, professor of international refugee law at Oxford University, said it was not possible to opt out of part of the European human rights convention.

Instead, the UK would have to pull out of the whole convention.