Mr Johnson was forced late on Sunday to admit he could not unite his warring party, seemingly surrendering to Rishi Sunak.
The former chancellor became the second candidate to declare their prime ministerial ambitions.
The former chancellor is the first candidate to be the next prime minister believed to have reached the 100-nomination threshold.
Having resigned in disgrace just six weeks ago, Mr Johnson was attempting an extraordinary comeback.
Former PM Boris Johnson was lagging behind, as was Commons Leader Penny Mordaunt.
The former prime minister cut short a family holiday in the Dominican Republic as the race to replace Liz Truss gathered momentum.
The former prime minister has flown back to the UK, with many in his party tipping him to return to power after Liz Truss dramatically quit.
Candidates need the support of 100 MPs to get on to the ballot on Monday.
The Commons Leader has indicated that Jeremy Hunt will stay on as Chancellor if she wins.
Sir James Duddridge says the ousted prime minister is preparing to stand to replace Liz Truss.