Actor Sir Tony Robinson, a former member of Labour's governing National Executive Committee, says he has quit the party over its current direction.
He said he was leaving after nearly 45 years because of Labour's stance on Brexit, its handling of anti-Semitism allegations and its poor leadership.
Sir Tony, 72, is best known for playing Baldrick in the comedy Blackadder.
The political activist has spoken at rallies for the People's Vote campaign, which is calling for a public vote on the final Brexit deal.
His decision comes as Labour lost seats in Thursday's local elections, with voters turning to smaller parties and independents.
Announcing his move on Twitter, Sir Tony said it was partly down to the party's "continued duplicity on Brexit".
He has previously written a tweet to deputy leader Tom Watson, saying: "Our party members are overwhelmingly in favour of a second referendum. To campaign on a platform of constructive ambiguity would be unprincipled, duplicitous and rather sinister."
Labour has refused to fully endorse a further referendum on Brexit - as supported by many ordinary members - instead saying it would do so under certain circumstances.
Sir Tony, who has frequently criticised Labour leader Jeremy Corbyn on Twitter, also raised the issue of anti-Semitism and swore when describing the leadership in his tweet.
Labour has been dogged by criticism of how it has handled allegations of anti-Semitism since Mr Corbyn became leader.
I’ve left the Labour Party after nearly 45 years of service at Branch, Constituency and NEC levels,partly because of it’s continued duplicity on Brexit, partly because of it’s antisemitism, but also because its leadership is complete shit.— Tony Robinson (@Tony_Robinson) May 3, 2019
The Time Team presenter, who campaigned at several general elections, served on Labour's National Executive Committee between 2000-04.
Labour did not want to comment on his departure.