Sajid Javid has endorsed Liz Truss in a damaging blow for Rishi Sunak’s campaign, warning that Sunak’s economic prescription would mean “sleepwalking into a high-tax, low-growth” economy.

In a difficult day for Sunak, the leadership hopeful doubled down on his warnings over inflation and mounted a fresh attack on Truss’s plans for immediate tax cuts, saying any prime minister who fuelled inflation would be handing the next election to Labour.

A poll from the Conservatives also gave a boost to Truss’s campaign, putting her 32 points ahead with party members, a day after YouGov showed similar results.

The endorsement of Javid again exposed the major divides in the two campaigns over economic growth on the eve of the Bank of England’s interest rates announcement.

Javid, who worked with Sunak in the Treasury and resigned on the same day, triggering the downfall of Johnson, said Truss was best placed to “reunite the party” and said a new approach to the economy was needed – a direct attack on Sunak.

“I fought for strong fiscal rules in our last manifesto,” Javid wrote in an article for the Times. “But the circumstances we are in require a new approach. Over the long term, we are more likely to be fiscally sustainable by improving trend growth.

“Only by getting growth back to pre-financial crisis levels can we hope to support the high-quality public services people rightly expect.”

Sunak and Javid were once considered close allies though came into conflict once Javid returned to the cabinet and clashed over health spending.

Sunak warned in comments released on Wednesday night that immediate tax cuts would be eaten by mounting inflation.

He said he would “make gripping inflation my number one economic priority” rather than tax cuts. “If we rush through premature tax cuts before we have gripped inflation, all we are doing is giving with one hand and then taking away with the other,” Sunak said.

“That would stoke inflation and drive up interest rates, adding to people’s mortgage payments. And it would mean every pound people get back in their pockets is nothing more than a down payment on rising prices.

“A policy prospectus devoid of hard choices might create a warm feeling in the short term, but it will be cold comfort when it lets Labour into No 10 and consigns the Conservative party to the wilderness of opposition.”

Meanwhile, Truss said her planned tax cuts would deliver “immediate help” to people this autumn.

“We cannot tax our way to growth. My economic plan will get our economy moving by reforming the supply side, getting EU regulation off our statute books, and cutting taxes.

“Modest tax cuts - including scrapping a potentially ruinous corporation tax rise that hasn’t even come into force – are not inflationary.”

Experts have warned that the UK’s annual inflation could go as high as 15% by the start of 2023, driven by sharp increases in energy prices pushing up the cost of living. The Resolution Foundation thinktank said previous forecasts were likely to be incorrect and that pressures on prices would last longer and be more intense.

Javid’s article directly challenged Sunak’s claim that tax cuts would be inflationary. “Some claim that tax cuts can only come once we have growth. I believe the exact opposite – tax cuts are a prerequisite for growth,” he said.

“Tax cuts now are essential. There are no risk-free options in government. However, in my view, not cutting taxes carries an even greater risk.”

He said the government was “sleepwalking into a big-state, high-tax, low-growth, social democratic style model which risks us becoming a middle-income economy by the 2030s with the loss of global influence and power”.

Both Truss and Sunak embarked on a day of ground campaigning before three major hustings and debates in three days. Truss’s campaign had been buoyed by two polls showing a surge in support, a day after she was forced to ditch a policy on regional pay boards after an outcry from MPs.

A spokesperson for Truss said her vision was “resonating with the members” and she would continue touring the country to meet them, saying she was “really enjoying the chance to get around the country”.

Sunak’s team said he had attended more than 60 events since the beginning of the campaign, speaking to 5,000 members personally. His team and MPs backing the former chancellor have said they do not believe Truss has as commanding a lead as the polls suggest.

“The YouGov polling is absolute nonsense: they have no data on our members, so they can’t weight it properly,” a Sunak-backing MP said.

However, MPs have voiced concern long-term over Truss’s ability to unite the party after her embarrassing U-turn on cuts to regional public sector pay.

Pointing to an interview given on Wednesday given by Brandon Lewis, where he was forced to defend the U-turn, the senior Sunak-backing MP said it had brought back bad memories.

“I’m afraid that whole episode yesterday was very continuity Johnson; and so was Brandon this [Wednesday] morning repeating all those things about ‘misrepresentation,’” the MP said.

“I got fed up with being told to go out and say things that were just not true, or nonsense, and I’m buggered if I’m doing that again.”