Boris Johnson's allies have raised concerns that he could be the victim of a Cabinet reshuffle as Theresa May declined to say he will stay on as Foreign Secretary after the election.
A friend of Mr Johnson told The Daily Telegraph that "clouds are gathering" and they are "worried" for his future if Mrs May secures a landslide election victory.
"If she has a massive majority she may not need him," the ally said. "Boris selling Brexit might not work anymore - if she has a massive majority she sells Brexit."
The Prime Minister said that Mr Johnson is doing a "great job" but declined to comment on whether he will remain Foreign Secretary following a series of briefings against him by Cabinet colleagues.
Mrs May also repeatedly refused to endorse a claim by Mr Johnson that Britain is prepared to launch a cruise missile attack on Syria if the Assad regime uses chemical weapons again.
The Foreign Secretary suggested the Prime Minister might not seek approval in a parliamentary vote before an air strike. Mrs May instead dismissed the prospect of Britain intervening as "hypothetical".
Asked whether Mr Johnson will remain Foreign Secretary, Mrs May said: "Boris is doing a great job and Boris and I and every Conservative candidate will be out there across the country fighting to win this election for the future of our country."
A senior Tory involved in the General Election campaign made clear that Mr Johnson is a "key member" of Theresa May's Cabinet and will play a prominent role in the campaign.
The Foreign Secretary took centre stage in the Conservative campaign as he accused Jeremy Corbyn of being a "mutton-headed old mugwump".
In a series of broadcasts Mr Johnson subsequently said it would be "very difficult for us to say no" if Donald Trump asked for assistance in air strikes against the Assad regime.
He told BBC Radio 4's Today programme: "If the US has a proposal to have some sort of action in response to a chemical weapons attack and if they come to us and ask for our support, whether it's with submarine-based cruise missiles in the (Mediterranean), or whatever it happens to be, it would be in my view, and I know this is also the view of the Prime Minister, very difficult for us to say no.”
Pressed on whether he would seek approval in the Commons for such a strike, Mr Johnson added: "I think that needs to be tested. As I said I think it would be very difficult for us to say no.
"How exactly we were able to implement that would be for the Government, the Prime Minister."
Mrs May played down the prospect of military intervention. She said: "This is a hypothetical issue because there is no proposal on the table for further strikes in Syria.
"What is important is that we look to see how we can bring about a solution in Syria that is going to lead to a strong and stable Syria. That means a political transition, it means political transition away from president Assad."
However Jeremy Corbyn, the Labour leader, warned that air-strikes against the Syrian regime would not "hasten" reaching a political solution.
He said: "We don't need unilateral action, we need to work through the UN. Above all we need to bend ourselves totally to getting a political settlement in Syria and allow the inspectors space to work. Allow them to make sure we know who did that terrible chemical weapons attack. The issue has to be finding a political solution."
* Theresa May faces a grassroots rebellion in her party against Westminster cronies being imposed on safe seat constituencies as preferred candidates for the election.
In Hornchurch and Upminster, east London, where Dame Angela Watkinson is stepping down, David Cameron’s former special adviser Shaun Bailey and Transport Secretary Chris Grayling’s policy adviser Simon Jones were put on the shortlist by Conservative Central Headquarters.
But the local association chose Julia Dockerill, a Conservative councillor in Tower Hamlets and chief of staff to Mark Field MP who was born and bred in Essex.
Sources suggested she had been chosen as a protest to keep out what were seen as CCHQ’s preferred choices.
Because of the short lead-in time to the election, CCHQ can shortlist preferred candidates in constituencies, but it is ultimately a matter for the local members to choose the winner.
Meanwhile in the Aldershot constituency being vacated by Sir Gerald Howarth, the local association had hoped to interview the high-profile Eurosceptic Daniel Hannan MEP as a potential candidate, only for CCHQ to present a shortlist of three other names.