The MP for Suffolk Central and Ipswich North, with a majority of 23,391 at the last election, has indicated he is not planning to stand at the next general election.

The defection was revealed in an article on The Observer website, in which the part-time GP outlined why he was switching parties.

He said: "The chaos of today's fragmented patchwork of community addiction services - making A&E the default location for people to get treatment and help - has added pressure to an already overstretched service.

"The mental toll of a service stretched close to breaking point is not confined to patients and their families. It also weighs heavily on my NHS colleagues who are unable to deliver the right care in a system that simply no longer works for our patients.

"It is this which has led me today to have resigned from the Conservative party to focus on my work as a doctor and to support Keir Starmer."

He told The Observer the Conservatives had become "a nationalist party of the right" in the last eight years.

"It is not to say all [Tory] MPs are like that," he said.

"There are good MPs, but it feels that the party is ever moving rightwards, ever presenting a more nationalist position."

The mental health doctor sent his resignation to the prime minister Rishi Sunak, saying: "After deep reflection and much heart-searching, I have decided, in all professional conscience, that I can no longer continue as a member of the Conservative Party."

The Labour leader Sir Keir Starmer celebrated his defection on social media.

"It's fantastic to welcome Dr Dan Poulter MP to today's changed Labour Party," he said in a post on X.

"It's time to end the Conservative chaos, turn the page, and get Britain's future back. I'm really pleased that Dan has decided to join us on this journey."

However, two councillors in Dr Poulter's constituency seem glad to see him go.

Councillor Samantha Murray said in a post on X: "We work so hard in North Ipswich as a Conservative group and have had to carry him for years."

Councillor Ian Fisher posted: "Was campaigning this morning and he didn't even have the decency to tell his hard-working activists in advance. A very self-centered man."

Sky's Jon Craig called the defection a "disastrous blow for the Conservatives and a massive propaganda coup for Labour."

Wes Streeting, Labour's shadow health secretary, said in a tweet: "Proud to welcome Dr Dan Poulter MP to the @UKLabour Party.

"As a frontline clinician, he's seen the damage that 14 years of Conservative government have done to our NHS.

"Delighted to have his support and look forward to working with him, especially on mental health reform."

A Conservative Party spokesperson responded to the resignation by saying: "For the people of Central Suffolk and North Ipswich this will be disappointing news. What Dan says is wrong as Sir Keir Starmer has no plan for our NHS.

"Under the Conservatives we are raising NHS funding to a record £165 billion a year, helping it recover from the effects of the pandemic and driving forward its first ever long-term workforce plan so that we train the doctors and nurses we need for the future in our country.

"Thanks to our plan, we have already virtually eliminated the longest waits and overall waiting lists have fallen by 200,000 in the last five months - and we will go further to make sure everyone gets the world-class care they need.

"This stands in stark contrast to the Welsh NHS - run into the ground by the Welsh Labour Government over the last 25 years which has waiting lists and waiting times way beyond what is being delivered in England."