Labour peer Lord Winston has accused the party of being "too frightened" to stand up against Brexit.
The broadcaster and fertility expert criticised Labour's decision to support Brexit, claiming that sometimes politicians had to have the "bravery" to defy public opinion.
He also slammed Labour leader Jeremy Corbyn's performance in comments to the House magazine.
Lord Winston said he felt "quite sick" when Mr Corbyn justified his decision to stand for the leadership.
And he alleged his allies, shadow chancellor John McDonnell and communications chief Seumas Milne, were like "puppeteers" controlling him.
He added: "I'm sure he's quite a decent man. In a way it's not his fault, he's a symptom of the malaise.
"I've been looking at his speeches from the 1980s. I can't see him getting better; he's been here a hell of a long time to improve."
His comments came as Labour MP Joan Ryan warned that the "terrible" defeat in the Copeland by-election showed the party was "well off the pace" needed to win a general election.