A group of Labour activists – who call themselves 'Labour for a Republic' (L4AR) – will hold a fringe conference event on Saturday.
They say the official mourning period between Queen Elizabeth II's death and her funeral on Monday was used as a "cloak" to calm opposition to King Charles III's accession to the throne.
Their conference event will be entitled "What future for the monarchy?" and feature Polly Toynbee, a columnist for The Guardian.
The event looks set to go ahead, despite Queen Elizabeth II only being laid to rest on Monday.
Sir Keir Starmer at Queen Elizabeth II's state funeral on Monday
JOHN SIBLEY
Queen Elizabeth II's funeral procession
Peter Nicholls
The move has sparked division in Sir Keir's ranks, as he plans to start conference by playing the National Anthem.
L4AR chairman Nick Wall is adamant that it will go ahead, saying the existence of the monarchy is "at odds with Labour's values".
He said: "The legitimate period of mourning has been used as a cloak to avoid any discussion about the accession.
"This is both coercive and insensitive. In a democracy, discussion about the monarchy is not something that should be shut down, it is a healthy public debate that should be encouraged.
"We have seen in recent weeks how increasingly divisive and out of touch the monarchy is.
"On the one hand we have millions of people in Britain living in fuel poverty, with millions more set to follow, as well as unprecedented numbers relying on food banks.
"On the other hand we have seen millions spent on royal parades, which have a purely political aim – to put in place a new Head of State without any scrutiny or consultation with the British people."
It could well be a source of frustration for the Labour leader, as Sir Keir has repeatedly spoken in tribute to Queen Elizabeth II since her death.
He was one of 2,000 mourners gathered in Westminster Abbey on Monday.
Sir Keir wrote: "Today marks the passing of an era.
"The dignity, courage, spirit, selflessness and good humour Her Majesty Queen Elizabeth II showed throughout her reign will always be with us. We are lucky to call ourselves Elizabethans."