The DUP has signalled it is likely to pull the plug on the deal that keeps the Tories in power if Rory Stewart becomes prime minister because he is refusing to renegotiate the Brexit deal.

The DUP’s confidence and supply deal that props up the government will expire at the end of June – and the party refused to comment on potential conditions for a renewal of the agreement.

But DUP sources indicated to HuffPost UK they would be unlikely to support a PM who remained committed to the backstop in Theresa May’s Brexit deal.

The Northern Irish party believes it will see the province is treated differently to the UK mainland and fears it could fuel demands for a united Ireland.

Stewart is the only candidate in the Tory leadership race who has not committed to try and either scrap or time-limit the backstop by attempting to reopen negotiations with the EU, arguing that Brussels has made clear the withdrawal agreement is closed.

Responding to this story, Stewart later told HuffPost UK he would sit down for “serious” talks to address the DUP’s concerns if he made it to the final two, and said unionism “is my soul” as a Scot living in England.

But rival campaigns said his plan to try and force through the deal unchanged was unrealistic because it was unclear how he could govern as a Tory PM without the support of the DUP, even if he managed to pass Brexit with Labour votes.

One DUP source said: “The problem is, he may be the best orator but he’s telling the wrong story, and the wrong story is that he wants to retain the backstop.

“How sad if the party continues on a pathway for three years that led nowhere to pursue that again.

“It has to be something different.” 

Conservative MPs backing different campaigns are already holding backchannel discussions with the DUP to try and reassure the party that its wishes will be taken into account by the incoming PM.

The DUP source said they had been making clear that: “All we want is the backstop done away with, or make it time-limited in a legally binding way.

“If you do that then we are honourable people, we will honour you.”

Another DUP source refused to comment on what could be on the table in confidence and supply talks that cold run into August.

But they indicated the party was relaxed on Brexit because Stewart is a rank outsider in the Tory leadership race.

“I don’t think there is any potential prime minister who is committed to retaining the backstop, they’ve all said no,” the source said.

“Rory Stewart is not going to be prime minister, let’s face it.

“All of the likely successful candidates have said the backstop has to go and we’re happy with that.”

Rival Tory leadership campaigns piled in on Stewart, who is battling to stay in the contest in the third round of voting on Wednesday afternoon.

One rival campaign source said: ’He’s just not realistic. Fourth time lucky with the withdrawal agreement is not a winning campaign slogan.

“He’s as divisive as Boris and we’ve got to offer the party more than a choice between marmite and vegemite.”

A source on another rival campaign added: “For a candidate who talks a lot about reality, it’s not entirely clear he’s attuned to the realities of parliament.”

Responding, Stewart told HuffPost UK: “I’ve been talking to Ian Paisley, we’d have to negotiate very closely with the DUP.

“The objective always that we were negotiating with… was to try and get an all UK deal.

“Their concern has always been about a border in the Irish Sea, and it is that we would have to work to avoid.

“But you are absolutely right that the backstop is a big concern for them, but no deal is also a huge concern for them.

“We would sit down and talk, I’m not setting any red lines, I’m trying to get a deal through.

“They, like me, care deeply about Northern Ireland’s place in the union, I am a passionate unionist, I’m a Scot representing an English constituency, I’m partially Irish, like them, this is my soul, so we’d work on it.

Stewart added: “You have to try and keep bringing people with you.

“And if I make it through to the last two they and I will have to sit down very formally, and I think they should have a serious conversation with both the final two contenders to what seems feasible.”