Labour Party leader Jeremy Corbyn launched his bid today to remain head of Britain's biggest opposition party despite bitter divisions that have been deepened by the Brexit referendum.

The veteran socialist has clung on even though three-quarters of the party's lawmakers say they have lost confidence in his leadership and Welsh MP Owen Smith has launched a challenge to oust him.

"It's the job, it's the duty, it's the responsibility of every Labour MP to get behind the party," Corbyn said at the start of his leadership campaign. "I hold out the hand of friendship," he said, addressing 'those who may not agree with me politically, may not even like me personally'.

Labour Party members and supporters will vote on Corbyn and Smith by postal ballot over the summer and the result will be announced on September 24.

Some commentators have warned the outcome could split the historic Labour Party, which grew out of Britain's trade union movement in the 19th century.

The 67-year-old Corbyn is automatically on the ballot as the current leader and he has strong grassroots support but Smith is backed by 162 of the party's 230 MPs including former leader Ed Miliband.

Some 180,000 Labour supporters have signed up to vote in recent days. "We, the Labour party, will be stronger, even stronger, and hopefully even bigger at the end of this campaign," Corbyn said.