Senior members of Britain's opposition Labour Party are planning to defy their new pacifist leader and vote with the government to carry out air strikes against Islamic State extremists in Syria, the Sunday Times newspaper reported.

The paper said that half of the party's shadow cabinet would side with Prime Minister David Cameron's Conservative government in a House of Commons vote it said could be held next month.

Britain is already taking part in US-led air strikes against the Islamic State group in Iraq and the government wants to extend the campaign to Syria but has said it will ask for parliamentary consent.

"There is a majority in the Commons for air strikes in Syria if Cameron has a proper plan for targeting Isis," a shadow cabinet member told the paper, using another acronym for the jihadist group.

"You would get half the shadow cabinet supporting it," the senior Labour official was quoted as saying.

The party's newly-elected radical leftist leader Jeremy Corbyn on Saturday resigned his role as chairman of the Stop the War campaign group that he helped found but restated his anti-war stance.

"In stepping down as chair, I want to make absolutely clear my continuing solidarity with the coalition and its work against wars of intervention," Corbyn said in a statement.

"It is now my job to lead the Labour Party, including in the struggle for peace and international justice, and that is demanding my undivided attention."

Stop the War was originally set up to oppose US-led intervention in Afghanistan following the September 11 attacks and went on to organise Britain's biggest ever rally against the 2003 invasion of Iraq.

The group is opposed to air strikes in Syria.

In 2013, Cameron lost a parliamentary vote that would have given the go-ahead for air strikes on Syria in what was seen as a major foreign policy setback, after Labour voted against the measure.

The Sunday Times report comes after a rocky first week in office for the 66-year-old Corbyn since he was elected Labour leader in a shock landslide victory announced on September 12.

He was voted in on a wave of anti-austerity sentiment in the party's grassroots, even though several Labour MPs see him as being too left-wing and unlikely to win a general election.

Corbyn has also said he backs the abolition of the monarchy and his refusal to sing the national anthem "God Save the Queen" in a World War II memorial was condemned by the right-wing press.