Ms Truss is being backed by Mr Johnson’s allies to face Rishi Sunak in the final two, but this depends on her overcoming Ms Mordaunt - who has received more votes than the Foreign Secretary in the first three rounds of the leadership contest.
But who is Tobias Ellwood, what happened in the confidence vote, and what has the reaction been?
Who is Tobias Ellwood?
Mr Ellwood was elected Conservative MP for Bournemouth East in May 2005.
He subsequently served as Parliamentary Under Secretary of State at the Foreign and Commonwealth Office from July 2014 to June 2017, before being appointed Parliamentary Under Secretary of State at the Ministry of Defence between June 2017 and July 2019.
In the 2019 leadership election, Mr Ellwood publicly endorsed both former Health Secretary Matt Hancock and former International Development Minister Rory Stewart.
Mr Ellwood ceased being a minister when Mr Johnson was elected as Prime Minister.
What happened with the vote of no confidence?
Mr Ellwood missed the confidence vote faced by the Government on Monday (18 July).
In a statement, the Bournemouth East MP said he had been unable to return to vote as a result of travel issues.
He commented: “Following my meeting yesterday with the president of Moldova, I was unable to secure return travel due to unprecedented disruption both here and in the UK.
“I am very sorry to lose the whip but will now continue my meetings in Ukraine promoting the Prime Minister’s efforts here and specifically seeking to secure the reopening of Odesa Sea Port – so vital grain exports can recommence.”
Mr Ellwood previously stated in February he would be sending a letter of no confidence in Mr Johnson to the 1922 Committee.
A source in the whips’ office said confidence votes needed to be taken extremely seriously by all MPs, with a failure to vote in support of the Government considered a failure to support the Conservative party agenda - regardless of whether Mr Johnson was leader.
Sir Graham Brady, the chairman of the 1922 Committee, confirmed that those who lose the Tory whip can no longer vote in the leadership race.
What has the reaction been?
As Mr Johnson’s decision comes just before the penultimate round of voting by Tory MPs today (19 July), it prompted speculation that he was working for the benefit of Ms Truss’s leadership campaign.
But Culture Secretary Nadine Dorries, who is considered a keen ally of the Prime Minister, angrily denied these claims.
She wrote on Twitter: “This is wholly untrue and frankly utterly ridiculous.
“Every single MP of every party is under no illusion regarding the price to be paid in not voting during a Gov confidence motion.
“It’s a very clearly defined and historic red line. Tobias could have voted like everyone else.”
A Tory source added: “Other Conservative MPs cancelled foreign trips, left poorly relatives, and one MP’s mother died on the morning of the vote - and they still attended and voted.”
Liberal Democrat Chief Whip Wendy Chamberlain said: “It is telling that Boris Johnson acted swiftly to punish Tobias Ellwood this time, but dragged his feet for days when it came to suspending the whip from his loyal supporter Chris Pincher.
“This petty act shows there is no room in the Conservatives any more for those who refuse to prop up Johnson.
“The Conservative leadership candidates should condemn this move and make clear they will restore the whip to Tobias Ellwood.”
In the confidence vote, which Mr Johnson called himself after the Labour Party sought one to remove the Prime Minister from office, MPs voted 349 to 238 - meaning a majority of 111 supported the Government.
Mr Ellwood’s name appeared on the no vote recorded list alongside 11 other Conservative MPs.
According to a Tory source, the other Conservative MPs on the list were appropriately paired with other MPs for the purpose of the vote, giving them a valid excuse not to.
Mr Johnson’s victory means he is expected to continue in Downing Street until September, when Tory members choose the winner from the final two leadership candidates.