Keir Starmer has been dealt a fresh blow after one of his top aides resigned over leaked messages where he made sexually explicit jokes about veteran MP Diane Abbott.

Paul Ovenden, Mr Starmer’s director of strategy, quit after it emerged that he had shared the derogatory comments with a female colleague while working in Labour's press team in 2017.

The loss of one of his closest advisers is a significant blow to the Prime Minister, who is battling to get on the front foot after he was forced to sack Peter Mandelson as US ambassador over his ties to Jeffrey Epstein.

The messages, leaked to ITV News, show Mr Ovenden telling a colleague about playing a game of "shag, marry, kill" at a party the night before. He said, "I nearly wet myself from laughing," before describing how partygoers had joked about a sex toy and a sexual act in relation to Ms Abbott.

A No 10 spokesperson said: "These messages are appalling and unacceptable. As the first black woman to be elected to Parliament, Diane Abbott is a trailblazer who has faced horrendous abuse throughout her political career. These kinds of comments have no place in our politics."

Mr Ovenden's departure has left his colleagues reeling, as he played a pivotal role in Labour's election victory, serving as director of communications from 2020 until last summer when he joined the No10 team.

One Government source told the Mirror: "Paul is the best brain in the Labour party. Everyone in No10, including the PM, relies on him as the voice of the people in the room." Another said: "His departure will hurt across Whitehall and in party HQ."

Mr Ovenden played a pivotal role in Starmer's election victory, serving as director of communications from 2020 until last summer when he joined the No10 team. He was previously a press officer for the party from 2014-2017.

In a statement today, Mr Ovenden said: "I am accused of eight years ago as a junior press officer sharing with a female colleague the details of a silly conversation that I was party to with other female staff members.

"Before summer, I had announced to some of my colleagues my intention to leave Government. Though the messages long pre-date my current employment and relationship with the Prime Minister, I've brought forward my resignation to avoid distracting from the vital work this Government is doing to positively change people's lives. As an adviser, my duty is to protect the reputation of the Prime Minister and his Government."

Ms Abbott, Parliament's longest serving female MP, has endured intense racism and sexism throughout her career. An Amnesty study found she was the victim of almost half the abuse directed at women MPs on Twitter during the 2017 election campaign.

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Labour MP Nadia Whittome expressed her horror at the messages. She said: "The misogynistic comments about Diane Abbott, the longest serving Black MP, recounted with glee by Paul Ovenden are utterly despicable."

It comes as the Prime Minister continued to face questions about his judgement over the appointment of Lord Mandelson when his friendship with Epstein was well known. He said on Monday he wouldn't have appointed the Labour grandee to the Washington role if he had known the depths of their relationship.

The PM publicly backed Lord Mandelson at Prime Minister's Questions on Wednesday - but sacked him the following day in the wake of a damning cache of leaked emails between him and Epstein.

Asked whether he was angry with Lord Mandelson, he told Channel 4 News: "I'm angry to have been put in that position."

The PM said: "In retrospect, of course I think it would be better if the detailed allegations that have been made in relation to Peter had been put in front of me before PMQs. But the team were trying to get answers out of him in relation to those questions and that was the right thing to do."

He rejected calls from some backbench Labour MPs for him to go, saying: "No, because I'm absolutely clear what the task is in front of me. We have a crossroads, really, in terms of the future of this country.

"We go forward with Labour for national renewal, a patriotic call about this country and taking this country forward, true patriotism, or we have division and decline under Reform."

Energy Secretary Ed Miliband said the Government should "stay focused on the job" after a period of "turbulence". Mr Miliband told BBC Radio 4’s PM programme: "If you talk about the last couple of weeks, we've gone through a period of turbulence, as governments do.

"The deputy prime minister resigned, and we've had the whole business with Peter Mandelson. I've been around a long time, as you know, I think there's only one answer to this, which is stay focused on the job. That's what I'm doing. That's what Keir is doing."

The Conservatives have secured an emergency debate in Parliament today(TUES) on the scandal, and will pile pressure on the PM to release documents relating to Lord Mandelson's appointment.