Mr Nowak has said the war of terror is undermining human rights

The measure was one of those announced by Tony Blair in the wake of the July bombings of London's transport network.

Agreements will be sought to make sure returnees would not be tortured.

But Manfred Nowak, UN special rapporteur on torture, said such assurances were "not an appropriate tool to eradicate this risk".

In the past Mr Nowak has said the war on terror is undermining human rights.

First agreement

The UK's plan reflected a tendency across Europe to circumvent international obligations, he added in a statement issued by the UN in Geneva.

That such assurances are sought shows in itself that the sending country perceives a serious risk Manfred Nowak, UN special rapporteur on torture

He said governments should not seek any kind of diplomatic assurance or memorandum of understanding as a means of avoiding their international obligation not to deport anybody at risk.

"The fact that such assurances are sought shows in itself that the sending country perceives a serious risk of the deportee being subjected to torture or ill treatment upon arrival in the receiving country," he said.

Earlier this month the UK signed the first agreement with Jordan.

The deportation measure was part of its response to the London attacks in July in which 52 people died and 700 were injured.

Criticism 'misplaced'

Ten people have already been detained and are facing deportation from the UK because the government says they pose a threat to national security.

Increased security measures were brought in after the bombings

Under the Human Rights Act, the UK cannot deport anyone to a country where they may face persecution.

Negotiations on similar agreements are now said to be taking place between the government and countries including Egypt and Algeria.

The Conservatives have backed the government's stance - but the Liberal Democrats say they would prefer to see detainees prosecuted in the UK.

Human rights groups have also expressed concern at the possible deportations, with Human Rights Watch describing Jordan's agreement not to mistreat deportees as "meaningless".

But Home Secretary Charles Clarke has insisted he has the necessary reassurances from Jordan that deportees would not be subject to torture or ill-treatment.

Writing in the Evening Standard newspaper on Monday, he said criticism was "entirely misplaced".

"Of course our agreements rightly include a guarantee of independent monitoring. They rightly seek to guarantee fundamental human rights," he said.

"We are living in unprecedented times, and it is time to break new ground in taking international action in dealing with the abhorrent threat of terrorism."