Immigrants should do community work to help them settle before being granted UK citizenship, Chancellor Gordon Brown has said.
Citizenship should be a contract, Gordon Brown believes
Mr Brown, widely expected to be the next prime minister, said citizenship should be a "kind of contract" with rights and responsibilities.
He said citizenship and language tests did not go far enough.
The Tories and Lib Dems accused him of "headline grabbing" and say it would be unenforceable or even dangerous.
The chancellor told an audience in London that obliging migrants to carry out community work would help introduce them to the people they will be living alongside and would show they could contribute to society.
He said he believed his earlier call for immigrants to be required to be able to speak English to be granted citizenship was now widely accepted, but more needed to be done.
Mr Brown said: "Being a British citizen is about more than a test, more than a ceremony. It's a kind of contract between the citizen and the country involving rights but also involving responsibilities that will protect and enhance the British way of life.
"It's also right to consider asking men and women seeking citizenship to undertake community work in our country, or something akin to that, that introduces them to a wider range of institutions and people."
He said a debate about what it means to be British was overdue, particularly in light of devolution, the EU Constitution, integrating minorities and dealing with Muslim fundamentalism.
Building a sense of national purpose was needed, to bring society together, he said.
He is also said to be considering whether citizenship should be granted on a trial basis, to be revoked if people did not keep their part of the contract.
In 2003, the radical Muslim cleric Abu Hamza al-Masri was stripped of his citizenship by then-home secretary David Blunkett, saying he wanted to deal with people the intelligence services believed to be a "risk".
But Sir Bernard Crick, a former Home Office adviser on citizenship, said they had suggested that immigrants do some sort of voluntary work - but that it had been rejected by the Treasury due to costs.
He added that any idea of a "contract" which could be revoked was "threatening" and would be unenforceable.
And Habib Rahman, of the Joint Council for the Welfare of Immigrants (JCWI), said: "Compulsory community service is usually imposed as a non-custodial penalty for a criminal offence.
"We are therefore extremely concerned that it is now being proposed as a condition of citizenship."
For the Conservatives, shadow home secretary David Davis said the proposal was ill thought out and might mean forcing some professionals who had been in Britain for years to stop working for a time to carry out community service.
He added: "This is a headline-grabbing initiative of very little substance. The problem is not with those applying for citizenship but with the number of illegal immigrants coming into the country."
Liberal Democrat leader Sir Menzies said: "This is just a gimmick, and would be impossible to enforce.
"We need proper provision for teaching English, not more headline chasing."