Reforms that could offer American citizenship to millions of illegal immigrants came under threat today as a brief peace between Democrats and Republicans on Capitol Hill fell apart.

Yesterday evening, Senators on both sides celebrated a rare compromise that would allow the passage of an immigration Bill that would give citizenship to illegal immigrants who have been in America for five years or more, and tighten security on the US-Mexico border.

But late last night, cracks in the Republican ranks started to appear and today the Bill was effectively delayed until after the two-week Easter break when the Senate voted overwhelmingly for amendments to be put forward to the legislation.

Democrat and Republican supporters of the Bill had vigorously resisted any amendments, saying that unless the legislation was passed in its present form, it was likely to unravel.

Even as it stood, the compromise agreed by Senate leaders last night differed markedly from legislation approved by the House of Representatives, and delicate negotiation would have been needed to reconcile the two versions. Now it is unclear when the debate will resume.

In a sign of the political importance of the Bill, which is felt to be a key vote-winner among America's growing Latino population, President George Bush urged the Senate to find a way through the impasse today. But politicians on both sides blamed each other for the stalemate.

Arlen Specter, a moderate Republican, accused Democrats of stalling the Bill to fan the flames of Latino protests, which have poured through Los Angeles, Chicago, New York and Denver in recent weeks. A huge demonstration is planned in Washington on Monday.

"It has not gone forward because there’s a political advantage for Democrats not to have an immigration bill," said Mr Specter.

Edward Kennedy, the Democrat Senator for Massachusetts and arch-foe of the Republican party, said that the Senate had "failed in our duty to our nation and our democracy and our American people".

"We certainly feel that we’ve lost an important chance and opportunity to make important progress on this issue," said Mr Kennedy. "For those committed to comprehensive immigration reform this debate certainly is not over, we’ll continue, if not today then tomorrow and the days ahead because the battle must go on."

Last night's compromise had appeared give form to the political voice of America's Latino population, the main agitators for immigration reform in a country that has around 11 million illegal aliens, a virtual underclass of undocumented manual and domestic labourers and volunteer vigilantes who patrol the US-Mexico border.

The sight of hundreds of thousands of angry Latinos taking to the streets of American cities, and the prospect of losing the support of the country’s fastest-growing ethnic group for a generation to come, had seemed to persuade Republicans to abandon their plans to make all illegal immigrants criminals.

The compromise was expected to restrict the flow of new arrivals and give seven million undocumented workers who have lived in the US for more than five years the right to apply for citizenship, provided they pay fines and taxes and learn English.

Three million others will be asked to go home before returning on temporary work visas that could ultimately qualify them for citizenship. About a million who have been in America for fewer than two years will be required to leave and then "wait in line" for permits.

Most Republicans had previously supported hardline legislation approved by the House of Representatives in December that made it a federal felony to live illegally in the US.

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