A former Labour chief whip has urged Jeremy Corbyn to "reflect" on Tony Blair's approach when party leader by ruling out the de-selection of MPs.
Baroness Hilary Armstrong told the BBC Mr Corbyn was "the greatest rebel ever" as a backbencher but Mr Blair was reluctant to discipline him.
She said the then prime minister felt that Labour was "a broad church".
Amid claims Mr Corbyn's opponents could be forced out, Baroness Armstrong said he needed to show he is "tolerant".
Speaking to BBC Radio 4's The Westminster Hour, Baroness Armstrong said she was pleased the Labour party chairman Ian Lavery had said de-selection was not the way forward.
But she added: "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."
She said "sectarianism" was "ruling" in some areas.
Baroness Armstrong added: "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?"
Mr Corbyn voted against his own government more than 500 times and Baroness Armstrong said at the time there was upset among party members in his Islington North constituency,
"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 de-select him?'".
She advised them they would not be supported by the leadership.
Baroness Armstrong said: "The prime minister was very clear about that when Jeremy was a backbench MP. And he was right, we shouldn't have worked to de-select 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."