The former chancellor George Osborne has accused Theresa May of failing to defend the Conservatives’ economic record and of presenting her party as “against the future” in June’s general election campaign.

Speaking to journalists in Westminster, Osborne, who was sacked by May last year and left parliament to become the editor of the Evening Standard, repeatedly criticised the prime minister’s leadership.

In particular, he accused May of abandoning the “compassionate Conservatism” he and David Cameron had sought to create – in part by facing down critics within their own party.

“In government, one of the decisions I am most proud of is the decision to legalise gay marriage – but the biggest opposition we faced was in our local Conservative associations and among the parliamentary party,” he said.

“These fights happen, but they’re worth having, and I think if we present ourselves to the country as anti-modern, anti-immigrant, anti-urban, anti-metropolitan, then huge sections of the country will be anti-us. We saw that frankly at the last general election and we may see it in the London elections in a few months’ time. Change in a progressive country is constant, and it’s pointless resisting it.”

Osborne urged the leadership to be more pro-business and internationalist. “If you as a party set yourselves against the future, if we’re hostile to business, if we think they’re the problem not the solution, if the cabinet game becomes who can get the most money out of the chancellor, if we’re anti-tech, if we talk about building homes but pretend they can only be built on brownfields, then we will lose our economic credibility and cause damage to our country’s future,” he said.

Asked whether he should bear any personal responsibility for the Tories losing their majority in June after Labour’s campaign focused on austerity, Osborne instead blamed May for failing to defend the government’s record.

“The decisions we took to try to get the budget into balance were hugely controversial at the time, and bitterly contested,” he said. “I think we proved that we could explain why they were necessary in the 2015 election, and there’s nothing that I can see that fundamentally changed between 2015 and 2017 – other than the Conservative party decided not to make the argument any more.”

The former MP for Tatton claimed Labour could have won the last election with a different leader, and described the changes to party leadership rules initiated by Ed Miliband as one of the “big choices” that shaped politics.

“For all his undoubted ability to connect to younger and more disillusioned voters, Jeremy Corbyn has become the biggest obstacle to Labour winning a general election,” Osborne said. “If the party was led by a more moderate social democrat of even middling ability, they would be 20 points ahead in the polls and on the cusp of power. Instead the Labour movement is consumed by an internal battle for its soul.”

Osborne hit out at May’s approach to leaving the European Union, accusing the Tory leadership of trying to drive through a hard Brexit.

“How we leave was not on the ballot paper. Whether we remain in the single market that Margaret Thatcher created or the customs union was not on the ballot paper. Whether we shut our country off to immigrants was not on the ballot paper.” He suggested the prime minister could find she did not have the votes for a hard Brexit.

Osborne said his newspaper – which he has frequently used as a platform to criticise May – would “expose the false arguments and broken promises of the hard Brexiteers.” He added: “The future lies with those who are able to bring the country together, not further its divisions.”

He said the Tory leadership had paid too little attention to the views of remain voters. “To say that 52% won and the 48% should shut up was a big mistake. I am hugely impressed that the Conservative party won in Mansfield and Stoke and Middlesbrough, which we never did when I was in charge of elections – but we lost in Bath and in Bristol and in Reading and in Brighton … you’ve got to be able to try to hold both.”