The debate will feature finance spokesperson Marra up against Fagan, a former council leader who was elected as an MSP for the South of Scotland in May.

They both met the threshold required to be eligible for the ballot to succeed Anas Sarwar, who quit just over two months after Scottish Labour suffered its worst Holyrood election result.

Sarwar has since been made a lord and has joined the Department for Business, Innovation, Science and Trade in Andy Burnham’s Cabinet.

He was a leading figure in the campaign for justice for bereaved families in the “dirty hospitals” inquiry centred on the Queen Elizabeth University Hospital and the Royal Children’s Hospital, both of which serve West Dunbartonshire and Argyll.

Monica Lennon, who lost her Holyrood seat in May and is taking up the list position vacated by Lord Sarwar, had hoped to stand in the contest but failed to secure the necessary support.

Marra, Fagan and Lennon, pictured left, took part in a hustings for Labour members last week before nominations closed.

Lennon has since been unable to secure the backing of three MPs to advance to the final run-off.

Marra, the author of the party’s manifesto ahead of May’s election, secured the support of 24 MPs and seven fellow MSPs, while Fagan has the backing of seven MPs and five MSPs.

Voting for the next Scottish Labour leader opens on August 28, and the new leader will be announced on September 19.