The Conservative rebel Anna Soubry has accused Theresa May of capitulating to “the forces of darkness” in her party and warned that, as a result, she and her fellow soft Brexiters may not back the final deal the prime minister obtains from Brussels.

The former minister accused Jacob Rees-Mogg’s faction of pursuing a hard Brexit that would cost “hundreds of thousands of jobs” and said they were “now running Theresa” because the prime minister was forced on Monday to accept their amendments to the customs bill.

In an interview with the Guardian, Soubry pledged to carry on fighting for a soft Brexit but conceded that the balance of power within her party had changed. An “ultimate elite, who are playing politics with real lives” had become dominant, she said.

Soubry’s intervention followed a dramatic week at Westminster in which Conservative factions have openly battled for control while May tried desperately to keep her job and negotiate Brexit.

On Monday, the Broxtowe MP gave a barnstorming speech during the debate on the customs bill, voicing anger at May’s decision to do a deal with Rees-Mogg’s European Research Group to head off a damaging rebellion. Until then Soubry had supported the prime minister’s new softer Brexit strategy that was signed off at Chequers a fortnight ago.

Reflecting on those events, Soubry said: “Since the referendum, the forces of darkness have taken over and they have continued to take over and this week if there was every any doubt who was running this government it was quashed.”

Facebook Twitter Pinterest Jacob Rees-Mogg in Whitehall. Photograph: Henry Nicholls/Reuters

The MP said she had been told by the Tory chief whip, Julian Smith, on Monday afternoon that “80 Conservatives, including a large number of members of the government” had told him they would not support the customs bill unless May accepted the ERG amendments.

That would have been enough to topple a flagship piece of Brexit legislation, and forced No 10 to agree to the amendments, one of which was designed to frustrate her proposals for a customs arrangement in which HM Revenue & Customs could collect some duties on behalf of the European Union.

The MP said she believed the ERG were “ruthless bastards” who would stop at nothing because they believed their cause was “bigger than anything else”. She added: “Now that the forces of darkness have got the prize in sight, they are going for it. I’m afraid that Theresa has embraced these people, they are now running Theresa.”

ERG members and supporters include a range of Tory backbenchers, from veteran anti-Europe campaigners, such as John Redwood and Bill Cash, through to former ministers David Jones and Steve Baker, and young MPs including Andrea Jenkyns, who want to force out May. Rees-Mogg is its chair.

Soubry accused May and party whips of operating a double standard, saying: “Jacob Rees-Mogg gets his tummy tickled when the ERG threatens to vote against a government bill unless it is amended on policy.”

In contrast, when her ally Dominic Grieve “suggested parliament should have a vote in the event of no deal, which is process”, he and his potential Tory supporters were treated differently. “You are threatened, you are called a traitor, you are threatened with deselection, threats the like of which have never been seen before,” she said.

Soubry warned the prime minister that she and her smaller group of about a dozen “mutineers” had not given up and could not be relied upon to vote for whatever the prime minister brought back from her Brexit negotiations.

“If she [May] stands a chance of getting her deal through she has to satisfy the mutineers, as we are called,” Soubry said. “I will vote on the basis of whether it protects jobs and prosperity for my constituents”.

The 61-year-old MP said her view of the tight parliamentary arithmetic was that May would have to rely on about a dozen or so Labour MPs to vote with her because she believed that a hardcore of Tory ERG members would not vote for whatever was proposed.

About five Labour MPs, including Kate Hoey and Frank Field, have consistently voted with the government on Brexit, but Soubry said more Labour MPs would be needed for May to see off any Tory rebellions: “She needs the likes of Caroline Flint to get it through.”

Play Video 1:51 Anti-Brexit Tory MP Anna Soubry asks: who runs the country? – video

Soubry conceded that the Tory remainer faction lacked the strength and discipline of the ERG up until now, adding that she and like-minded colleagues “needed to grow a pair” because they had failed to pull out or pulled back from the brink when confronting the government on Brexit.

Soubry said the remainers needed to muster 16 rebels if they were to defeat the government, assuming all the opposition parties voted alongside them. “We did have at least 16 when the EU withdrawal bill came back, and the big mistake was that we did not do what we said we would do, which was have a vote on the Kerr amendment, which was very tame. That is recognised as being a mistake.”

If passed by MPs last month, that amendment, proposed by the former diplomat now crossbench peer John Kerr, would have forced ministers to explain what they had done to pursue remaining in a customs union.

Despite her frustrations, Soubry said it would be “deeply irresponsible” for anybody to challenge May and accused the prime minister’s rival Boris Johnson of “naked, blind ambition”.She said the former foreign secretary and his supporters were engaged in “willy-waving”, making light of the consequences of Brexit. “This is deeply grownup, serious stuff. Egos and this sort of willy-waving ambition is deeply unacceptable.”

The MP, who was narrowly re-elected with a majority of 863 last year, said she had no intention of leaving the Conservative party, although she cooperates closely in parliament with pro-European Labour backbenchers Chuka Umunna and Chris Leslie on Brexit. “I shall be true to what I believe. It is my intention to stand again in Broxtowe as a Conservative candidate. I just want my party back, please.”