The outcome of the leadership campaign battle will be announced on 5 September, meaning there is now less than a week until the winner is revealed.
Before this, Tory party members must decide on their favourite candidate - with voting set to end on 2 September.
UK bookies will therefore be preparing to pay out on the contest, as thousands of people flock to sites to place their bets.
But which PM hopeful is ahead in the polls, and what are the best odds?
Here’s what we know.
Who is most likely to win the Tory leadership race?
Ms Truss is the bookies’ favourite to become the next leader of the Conservative Party.
This may have come as a surprise to some, considering the former chancellor came out on top in all votes by MPs as the field of contenders was narrowed down.
Ladbrokes currently has the Foreign Secretary’s odds to win at 1/20, while Mr Sunak lies behind at 9/1.
According to bookies.com, this puts Ms Truss’ implied probability at 95.2% - and Mr Sunak’s at just 10%.
Similarly, betting website gambling.com places the South West Norfolk MP’s percentage chance at 97.09%, compared to 6.67% for Mr Sunak.
A little over a week ago, a YouGov poll of Tory party members placed voting intention for Ms Truss at 66%.
Mr Sunak was behind again, with just 34% of voters saying they would pick the Richmond MP to be Prime Minister.
These results mimic an Opinium survey from a few days earlier, which put the Foreign Secretary as receiving 61% of the votes and the former chancellor at the lower 39%.
Who decides the winner?
Conservative Party MPs initially voted on the candidates for the Tory leadership race, eventually narrowing the field down to just Mr Sunak and Ms Truss.
However, the decision now lies in the hands of Tory party members - who must place their votes before 2 September, either by postal ballot or online.
According to YouGov, as of 19 August, 57% of the party electorate have already cast their vote, while 38% are still intending to do so.
This means that 5% may not cast a vote at all.
When will the next Prime Minister be announced?
The winner of the Conservative Party leadership race, and therefore the person who will become the UK’s next Prime Minister, will be revealed on 5 September.
The announcement will take place at 12.30pm in Westminster, in front of MPs and Peers.
Although it seems likely at this point that Ms Truss will be the one taking residence at Number 10, we will not know for definite until Monday.
Mr Johnson is expected to leave office on September 6, with a spokesperson adding that he will go to Buckingham Palace to tender his official resignation to the Queen.