His sudden exit deals a major blow to moderates who had fought the party leadership over Labour’s stance on Brexit and its handling of antisemitism.

It came at the end of a dramatic first day in the general election campaign that also saw the Welsh Secretary resign from Boris Johnson’s cabinet.

A frequent critic of Mr Corbyn’s leadership, Mr Watson faced an attempt to oust him at the Labour Party conference.

Labours deputy leader Tom Watson with Jeremy Corbyn. Picture: AP

There were reports last night that the news could lead to further walkouts by Labour moderates fearing a heavy defeat in next month’s election.  A Scottish Labour source told the Scotsman: “It’s a massive blow. Moderates up here are despairing.”

In a letter to the Labour leader, the West Bromwich East MP he said that the time had come to “start a different kind of life”. “The decision is personal, not political,” he said.

Mr Watson said he would campaign on public health issues after losing seven stone through dieting and exercise, and reversing his type 2 diabetes.

“The last few years have been among the most transformational of my personal life, second only to becoming a proud father of two beautiful children,” he said. “I’ve become healthy for the first time and I intend to continue with this work.”

Mr Watson added: “Our many shared interests are less well known than our political differences, but I will continue to devote myself to the things we often talk about: gambling reform, music and arts, stopping press intrusion, obesity and public health and of course horticulture and cycling.”

In his reply, Mr Corbyn wrote: “Few people have given as much to the Labour movement as you have and I know many thousands of members and trade unionists you have inspired and worked with over the years will be very sorry to see you go.”

Mr Watson came under pressure recently over his support of historical child abuse claims which were later found to be false.

Deputy leader Tom Watson rocks Labour with shock exit