Gordon Brown has praised the Archbishop of Canterbury's "great integrity" after criticism of his remarks on Sharia law.
Dr Rowan Williams will address the Anglican general synod
The prime minister's spokesman said Dr Rowan Williams had clarified his comments that implied adopting aspects of Sharia in the UK was "unavoidable".
But Dr Williams faces pressure from some Church of England traditionalists who want him to apologise.
The archbishop is expected to address clergy on the subject at the general synod later.
The prime minister's spokesman said Mr Brown understood "the difficulties" the archbishop was facing and paid tribute to Dr Williams's "dedication to public and community service".
Earlier the Times reported that Mr Brown had telephoned the archbishop and encouraged him to clarify his remarks where he felt they may have been misinterpreted.
Mr Brown's spokesman would not say if the two men had spoken over the weekend, but described their relationship as "close", and said Mr Brown had "been kept informed on this debate throughout".
Mr Brown believed religious law should be subservient to UK law, he added.
If the archbishop fails to apologise, 150 traditionalist members of the synod are believed to be prepared to sign an open letter distancing themselves from his suggestions.
BBC religious correspondent Robert Pigott said traditionalists, who believe Dr Williams should be fighting to preserve the Christian, biblical basis for the law, had "declared war" over the issue.
Dr Chris Sugden, canon and executive secretary of the evangelical organisation Anglican Mainstream, is one of those calling for an apology from the archbishop.
"Well we're saying that he's made a mistake - that he has caused great difficulties for our colleagues especially in Nigeria, especially in other countries where there's significant Muslim pressure for Sharia to come in," he told BBC Radio 4's The World at One.
He added: "We hope that he will have the integrity to say 'I made a mistake in what I said', and 'I'm sorry to those for whom it has caused great distress'."
The Reverend Rod Thomas, chairman of Reform, a group representing traditionalists, also told the BBC's One O'Clock News the archbishop's remarks had inflicted huge damage to Christians both at home and abroad.
"He has let down the Church and in particular I think he's left Christians in other countries that are trying to sound cautionary notes about the spread of Sharia law in an extremely difficult position."
HAVE YOUR SAY
No religion what so ever should dictate or influence laws or policy in Britain. Ever.
Daniel Burnett, Newcastle
Dr Williams sparked a major row after saying, in a BBC Radio 4 interview last week, that the adoption of parts of Sharia law was "unavoidable" in Britain.
He has insisted he was not advocating a parallel set of laws but has faced calls for his resignation.
Dr Williams's remarks were earlier criticised by his predecessor as the Church of England's leader, Lord Carey.
On Sunday, he said the acceptance of some Muslim laws would be "disastrous" for Britain.
But, writing in the News of the World, Lord Carey said his successor should not be forced to quit.
Two synod members have called for Dr Williams to stand down following his remarks.
Colonel Edward Armitstead, a member from the diocese of Bath and Wells, said Dr Williams should move to work in a university setting instead of leading the Anglican Church.
Alison Ruoff, a synod member from London, said he was "very able, a brilliant scholar as a man" but in terms of being a leader of the Christian community "he's actually at the moment a disaster".
However, our correspondent said it was "pretty inconceivable" that Dr Williams would resign.
Justice Minister Michael Wills told The World at One he understood why people felt strongly about "questions of national identity" and that some of the issues raised by Dr Williams were to be covered by a government review on Britishness.