former PM Boris Johnson's withdrawal means the contest could be decided by early afternoon today unless both the remaining candidates, including Rishi Sunak and Penny Mordaunt, can get the support of 100 MPs.

The former prime minister claimed he had the nominations needed to make it onto the ballot paper but admitted he could not unite his warring party.

In a statement on Sunday evening, Johnson said there was a "very good chance" he could have been back in No ten by the end of the week if he had stood.

However, his efforts to "reach out" to his rivals – Rishi Sunak and Penny Mordaunt – to work together in the national interest had not been successful, so he was dropping out.

While Sunak, the former Chancellor, already has more than 140 public declarations of support, Ms. Mordaunt, the Leader of the House, had fewer than 30.

Her team is now hoping that the departure of Johnson will see a swathe of MPs who were backing him or are yet to declare swing behind her.

A campaign source confirmed she was still in the running, arguing she is the candidate who Labour fears the most.

However, one senior minister backing Mr. Johnson – the Duchy of Lancaster Nadhim Zahawi's Chancellor said he would now be supporting Mr. Sunak.

"Rishi is immensely talented, will command a strong majority in the parliamentary Conservative Party, and will have my full support and loyalty," he tweeted.

With nominations closing at 2 pm on Monday, Ms. Mordaunt has limited time to get the necessary nominations. If she fails, Mr. Sunak will be declared Leader without a contest.