Deep divisions over Brexit overshadowed the opening day of the Conservative party conference on Sunday as Theresa May attempted to wrestle back the focus on to her domestic agenda.

The bitter infighting that has crippled the Conservative party was laid bare as Boris Johnson and Jacob Rees-Mogg laid into the prime minister’s Brexit plans as thousands of delegates gathered in Birmingham.

The chancellor, Philip Hammond, launched a scathing attack on Johnson, suggesting the former foreign secretary could not do “grown-up politics” and saying he did not expect him to become prime minister.

May appealed to Tory MPs and the party’s grassroots to back her Chequers proposal as she was forced to hit back at Johnson, her former foreign secretary, who questioned her belief in leaving the European Union.

“I do believe in Brexit, but crucially I believe in delivering Brexit in a way that respects the vote and delivers on behalf of the British people, while also protecting our union, protecting jobs and ensuring we make a success of it,” she told the BBC’s Andrew Marr.

However, May risked infuriating the party’s pro-Brexit grassroots by appearing to refuse to rule out further compromises to her Chequers plan in order to broker a final deal.

It came after Johnson used a newspaper interview to launch a renewed attack on May’s entire Brexit plan, dismissing it as “deranged” while suggesting the proposal for Britain and the EU to collect each other’s tariffs was “entirely preposterous”.

Rees-Mogg, the leader of the hard Brexiter European Research Group, said the plan was the “deadest of dying ducks” at a packed fringe meeting with hundreds of delegates; while Tory MP Conor Burns, a former aide to Johnson, also pleaded with the prime minister to change course.

Jeremy Hunt, the foreign secretary, who is often considered a leadership contender, struck a more combative tone in his speech to the party faithful, seeking to burnish his credentials with an attack on Brussels’ negotiating tactics.

He said the EU seemed to want to “punish” a member for leaving, and likened their tactics to the Soviet Union.

“The lesson from history is clear: if you turn the EU club into a prison, the desire to get out won’t diminish – it will grow … and we won’t be the only prisoner that will want to escape,” he said.

While Hunt remained loyal to May’s Brexit position, he went on to attack the EU for embarrassing the prime minister at the last summit at Salzburg with the unexpected rejection of Chequers.

“Because if you put a country like Britain in a corner, we don’t crumble. We fight. So as your friends of many years we say simply this: Brexit is not about whether you succeed or we succeed. Europe prospers when we both succeed and it’s time to change your approach.”

Latvia’s ambassador to the UK, Baiba Braže, in response to a tweet highlighting Hunt’s comparison of the EU to the Soviet Union, said: “Soviets killed, deported, exiled and imprisoned 100 thousands of Latvia’s inhabitants after the illegal occupation in 1940, and ruined lives of three generations, while the EU has brought prosperity, equality, growth, respect.”

However, several senior Tories criticised Johnson over his intervention. Scottish Conservatives leader Ruth Davidson said: “Now, I don’t sit around the cabinet table, I’m not in government … but I knew what was being said in December. I’m not quite sure how the former foreign secretary didn’t.”

Hammond was critical in his assessment of Johnson’s record and capabilities. Asked by the Daily Mail whether Johnson could become prime minister, Hammond said: “I don’t expect it to happen.” He added: “Boris is a wonderful character, but he’s never been a detail man. I’ve had many discussions with him on Brexit.”

Of their conversations about a “Canada”-style trade deal, Hammond told the paper: “Boris sits there and at the end of it he says, ‘Yeah but, er, there must be a way, I mean, if you just, if you, erm, come on, we can do it, Phil, we can do it. I know we can get there ... We just have to want it a bit more, we just have to wish a bit harder, we just have to be a bit more bullish and it will all be fine.’ It won’t all be fine because we are dealing with grown-ups here and we have to deal with the real world situation we face.”

The Tories have been desperate to show that they have more to offer the public than just infighting and will on Monday attempt to shore up their domestic agenda with a series of announcements, including plans to expand apprenticeships and detailing an eye-catching ban on employers pocketing workers’ tips.

Hammond will on Monday make the case for “21st century capitalism” as the Conservatives seek to fight back after Jeremy Corbyn’s most successful Labour conference yet unleashed a wave of concern among Tory ministers.

The chancellor will tell the party faithful that the best way to tackle years of slow wage growth, job insecurity and spiralling housing costs was a return to core Tory belief in “the power of enterprise as the route to unleash talent and to improve lives”.

Tory activists cheered when former CBI director-general Digby Jones, who is not a Conservative but was giving a business perspective on Brexit, joined in attacks on Boris Johnson over his “fuck business” comments this summer.

“Business is so important that when I heard a former foreign secretary [say] eff- business, it showed him up for the irrelevant and offensive person he really is,” Lord Jones told delegates.

Hammond was expected to tell delegates: “Too many people have experienced years of slow wage growth, felt less secure in their jobs and seen the housing market spiral beyond their reach. And as they look around them, they feel a growing concern that they are falling behind.

“So the challenge is to ensure that 21st century capitalism delivers for them; to convince them that our vision of Britain’s future can meet their aspirations and that our plan, unlike Labour’s, will actually deliver a better tomorrow for them and their families.”

He will add: “Economic freedom goes hand in hand with political freedom and above all, the belief in the power of enterprise as the route to unleash talent and to improve lives. That’s why we back business, as the cornerstone of a successful economy; as a force for good in our society; and as an essential expression of our values.”

The business secretary, Greg Clark, said on Sunday that the government’s determination to protect business underpinned May’s Chequers plan for post-Brexit trade.

“We are a country that is superbly well positioned in most of the big transformations that are taking place across the world today,” Clark told a HuffPo fringe meeting.

“To do that, we need to be able to build on the foundations that are successful – from manufacturing to pharmaceuticals and life sciences. And the ability to trade without introducing new barriers and frictions is important to that.”

Clark dismissed the “super-Canada” plan being pushed by Johnson and his fellow hardline Brexiters. He said: “One of the problems of the Canada model is that it requires frictions at the border, it requires checks at the border – I think everyone recognises that, and it doesn’t do what is necessary to avoid those frictions which would set back our competitiveness.”