Tony Blair ruled out a moved to deselect serial rebel Jeremy Corbyn because he believed the party was a broad church that should tolerate different opinions, according to a former chief whip.
As a backbencher, the now Labour leader voted against his own government more than 500 times and there was upset among the rank and file in his Islington North constituency, said Baroness Armstrong.
But the then Prime Minister was "very clear" that he would not support such a move, the Labour peer insisted.
It comes amid continued speculation that Mr Corbyn's opponents in the parliamentary party will face deselection.
Baroness Armstrong urged Mr Corbyn to "reflect" on Mr Blair's approach and make sure oustings are not carried out on his watch.
The peer told Carolyn Quinn on BBC Radio 4's The Westminster Hour that Mr Corbyn was "the greatest rebel ever" but Mr Blair was reluctant to discipline him as he believed the party could "tolerate that level of difference".
"I had a couple of folk from Jeremy's constituency come to see me and say, 'People are a bit upset with Jeremy always being against the Labour government, what if we try to deselect him?'".
But she advised them: "You will get no support from the leadership, so don't bother."
She added: "The Prime Minister was very clear about that when Jeremy was a backbench MP. And he was right, we shouldn't have worked to deselect him. But I hope that Jeremy will now reflect on that and I hope that he will be absolutely determined to make sure it doesn't happen under his watch."
Mr Blair has been a vocal critic of Mr Corbyn since he took the helm of the party and suggested that if people say their heart is with the Labour leader they should "get a transplant".
A toxic war between the factions in the party has led to claims that opponents of the leadership will face being deselected or forced out.
Baroness Armstrong said: "I know MPs where basically there is a process of harassment, where at every meeting they are criticised, they are challenged, they are told that they don't represent the people in the room. And all this is meant to do is grind them down, is wear them down, and get them to believe they shouldn't be in the Labour party any more.
"The real issue is, can you make sure that sectarianism doesn't rule? And at the moment in some areas, it is ruling. And Jeremy has the opportunity over the summer and at party conference to make it absolutely clear that he is not going to lead a narrow sectarian faction, he's going to lead a broad church that is tolerant. And the real test for Jeremy is, is he up to it?"