LABOUR has been rocked by the bombshell announcement that Tom Watson, its deputy leader, is leaving frontline politics to “start a different kind of life”.

On a day that had been dominated by Tory woes with the resignation of Alun Cairns, the Welsh Secretary, overshadowing the Conservatives’ election launch, the focus quickly switched last night to Labour and the departure of the party’s second most senior figure.

"The decision is personal, not political," declared Mr Watson, who has been his party’s deputy leader for more than four years.

HeraldScotland: Camley's Cartoon: Tom Watson resigns.Camley's Cartoon: Tom Watson resigns.

At last week’s Labour election campaign launch, the frontbencher cut a rather despondent figure among the cheering crowd in Battersea.

Last night, he announced his decision publicly on Twitter, writing: “After 35 years in full-time politics, I've decided to step down and will be campaigning to overcome the Tory-fuelled public health crisis. I'm as committed to Labour as ever. I will spend this election fighting for brilliant Labour candidates and a better future for our country.”

His announcement will be greeted with delight by some on the left of the party, most notably Len McCluskey, the Unite leader, with whom he clashed many times in recent years; the trade union chief, once a close friend, believed Mr Watson had become disloyal to Jeremy Corbyn.

However, many on the centre right will be saddened at the departure of a figure they believed was key to maintaining Labour’s appeal to those in the centre ground.

In a letter to Mr Corbyn, Mr Watson wrote: “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.

"I've become healthy for the first time and I intend to continue with this work in the years to come."

Mr Watson said that he would remain as deputy leader until polling day in December and would be playing an "active part" in Labour's election campaign.

The MP for West Bromwich East since 2001 was elected to his deputy leadership position at the same time in 2015 as Mr Corbyn was elected leader.

During their time together at the top of the party the two men clashed repeatedly with Mr Watson, a former ally of Gordon Brown, becoming a focus for the "moderate" opposition in the party to Mr Corbyn.

He was publicly critical of the leadership's attempts to tackle anti-Semitism in the party and led moves to push it into supporting a second referendum on the EU, despite the entrenched resistance of the leader.

Most recently, he defied Mr Corbyn by calling for the party to back a new public vote before the country went to the polls in a general election.

In June, as he campaigned for Labour to back the holding of a second referendum, he famously said: “I am never going to leave the Labour Party,” before adding: “Sometimes I wonder whether the Labour Party is leaving me.”

His opposition to Mr Corbyn angered allies of the Labour leader and on the eve of the Labour Party conference in September, they made an unsuccessful attempt to oust him by abolishing the post of deputy leader.

In a cordial exchange of letters, Mr Watson told Mr Corbyn: “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 response, the party leader said: “Few people have given as much to the Labour movement as you have and I know that many thousands of members and trade unionists that you have inspired and worked with over the years will be very sorry to see you go.”

Mr Corbyn pointed out how he had enjoyed “convivial” chats with Mr Watson, including about their shared love of gardening. “I hope the horseradish plants I gave you thrive,” he added.

In recent months, Mr Watson faced intense criticism over his role in promoting the false claims of a Westminster paedophile ring made by the fantasist Carl Beech.

Beech's allegations led to a number of public figures coming under police investigation, including the the late Lord Brittan, the former Home Secretary, Lord Bramall, the former head of the armed forces, and Harvey Proctor, the ex-Tory MP.

After facing calls to resign by some of those involved, Mr Watson said he was "very, very sorry" for the way events had turned out.

Last night, Mr Proctor said: “By standing down Tom Watson has done his constituents a great favour. The next parliament will be a healthier place without him.”