Labour leader Jeremy Corbyn is to address the annual conference of Blairite group Progress on Saturday.
Progress has been described by shadow chancellor John McDonnell as a “right wing conservative” group who had never accepted Mr Corbyn as Labour leader.
Mr Corbyn will address its members, which include some of Labour’s best-known MPs, at their conference in central London .
Labour’s leader in Scotland, Kezia Dugdale, is also due to speak.
Speaking in January, after several resignations from the Labour front bench, Mr McDonnell said: “They do all come from a narrow right wing clique within the Labour Party, based around the organisation Progress largely. I don’t think they’ve ever accepted Jeremy’s mandate really.”
He added that the group had “a right wing conservative agenda” and there were some within it “who are quite hard right”.
Progress, which was founded by Lord Mandelson to promote New Labour ideas, backed heavily-defeated Blairite candidate Liz Kendall in last year’s Labour leadership contest. Ms Kendall is among the other speakers on Saturday.
Ed Miliband, Gordon Brown and Tony Blair all gave speeches at Progress annual conferences when they were party leader. Mr Corbyn is not thought to have attended the event before.
BBC political correspondent Iain Watson said it was interesting that Mr Corbyn was meeting some of his fiercest critics – some of whom have been talking privately about trying to get rid of him as leader.
It could be seen as a sign that he is reaching out to his critics in the party rather than retreating into his comfort zone, he added.
But although the party is not briefing the contents of his speech, he is likely to repeat his tough message that MPs should not be giving a running commentary on his leadership.
Progress has borrowed Mr Corbyn’s “straight talking, honest politics” slogan for its 2016 conference in a further sign it is offering an olive branch to the Labour leader.