The Archbishop of Canterbury tonight prompted criticism from across the political spectrum after he backed the introduction of sharia law in Britain and argued that adopting some of its aspects seemed "unavoidable".

Rowan Williams, the most senior figure in the Church of England, said that giving Islamic law official status in the UK would help achieve social cohesion because some Muslims did not relate to the British legal system.

However, the prime minister's spokesman swiftly rejected the archbishop's comments, which were delivered in a lecture on civil and religious law at the Royal Courts of Justice.

Gordon Brown's spokesman insisted British law would be based on British values and that sharia law would present no justification for acting against national law.

"Our general position is that sharia law cannot be used as a justification for committing breaches of English law, nor should the principles of sharia law be included in a civil court for resolving contractual disputes.

"If there are specific instances like stamp duty, where changes can be made in a way that's consistent with British law and British values, in a way to accommodate the values of fundamental Muslims, that is something the government would look at."

The Conservative peer and shadow minister for community cohesion and social action, Sayeeda Warsi, also criticised the Anglican primate.

"The archbishop's comments are unhelpful and may add to the confusion that already exists in our communities ... We must ensure that people of all backgrounds and religions are treated equally before the law.

"Freedom under the law allows respect for some religious practices. But let's be absolutely clear: all British citizens must be subject to British laws developed through parliament and the courts."

However, some Muslim groups supported Dr Williams' views on sharia law, which sets out a broad code of conduct for all aspects of life, from diet to the wearing of the hijab.

The Ramadhan Foundation, an educational and welfare body, said the speech was "testament to his attempts to understand Islam and promote tolerance and respect between our great faiths".

More than 800 people were present in the Great Hall of the Royal Courts of Justice for the speech. A further 200 poured into the overspill marquee. Plasma screens were erected to ensure people could hear and see Williams clearly, and the audience was encouraged to introduce themselves to those nearby.

Williams said introducing sharia law would mean Muslims would no longer have to choose between two systems.

"If what we want socially is a pattern of relations in which a plurality of diverse and overlapping affiliations work for a common good, and in which groups of serious and profound conviction are not systematically faced with the stark alternatives of cultural loyalty or state loyalty, it seems unavoidable."

He compared the situation to faith schools, where "communal loyalties" were brought into direct contact with wider society, leading to mutual questioning and mutual influence towards change, without compromising the "distinctiveness of the essential elements of those communal loyalties".

Earlier, in a BBC interview, he was more succinct. He said it was a "matter of fact" that sharia law was already being practised in Britain.

"It's not as if we're bringing in an alien and rival system; we already have in this country a number of situations in which the internal law of religious communities is recognised by the law of the land ... There is a place for finding what would be a constructive accommodation with some aspects of Muslim law as we already do with some kinds of aspects of other religious law."

He did not endorse the "kind of inhumanity" associated with sharia law in some Islamic states.

The Bishop of Rochester, Michael Nazir-Ali, warned last month that attempts were being made to give Britain an increasingly Islamic character.

"There is pressure already to relate aspects of the sharia to civil law in Britain," he said. "To some extent this is already true of arrangements for sharia-compliant banking but have the far-reaching implications of this been fully considered?"

The bishop, who is no stranger to controversy, also claimed that extremists have created "no-go" areas, which were too dangerous for non-Muslims to enter. He has since received death threats and was placed under police protection. He was unavailable for comment today.