The Financial Times reported that while campaign insiders said the decision was not “100 per cent confirmed” Mr Sunak - who was an early backer of Mr Johnson for PM - was the favourite to stand in, as it was signalled the Conservatives did not see the event as crucial.

The FT quoted a Tory official who said Mr Johnson “really rates” Mr Sunak, who is standing for re-election in the Yorkshire seat of Richmond.

The official said: “He has great form from the leadership campaign and has really proven himself since joining the government. Boris is also keen to show there is new talent coming to the fore in the party.”

Chief Secretary to the Treasury and Tory candidate for Richmond, Rishi Sunak. Photo: Gary Longbottom

Mr Johnson is still set to appear in a head-to-head debate with Jeremy Corbyn tonight, as well as one further debate in the future and a Question Time special.

The situation echoes a similar BBC debate in 2017, when Theresa May sent Amber Rudd in her place.

Meanwhile Nicola Sturgeon, who will take part in Friday’s show, branded Boris Johnson a "scaredy-cat" over his refusal to debate her, saying she would face him "any time, any place".

The First Minister's challenge follows the SNP's exclusion from tonight's televised head-to-head debate between Jeremy Corbyn and Mr Johnson - a format Ms Sturgeon declared was "deeply undemocratic".

Fiona Bruce will host a Question Time Leaders’ Special, broadcast from Sheffield on BBC One and BBC Radio 5 live on Friday.

Yorkshire's Rishi Sunak lined up to replace Prime Minister in televised debate