The former education recovery commissioner, Sir Kevan Collins, has accused the government of burying its head in the sand over the loss of learning among children in England due to Covid, warning the problem will not just “go away”.

He expressed regret about lost opportunities after the government rejected his ambitious £15bn plan for recovery, including an extended school day for all, and warned that the flagship national tutoring programme (NTP) was in danger of becoming little more than “a few kids in the corner doing a bit of tutoring”.

Speaking a year after Covid restrictions were finally lifted on what the government called “freedom day”, Collins voiced concern about a tax-cutting arms race in the Conservative leadership contest, which he warned would result in cuts to education spending.

Collins resigned from his job as education catch-up tsar in June last year in protest at the prime minister’s decision to scale back recovery plans, warning the new offer did not come close to meeting the needs of children whose education was thrown into chaos by the pandemic. The government has so far pledged around £5bn in catch-up funding for schools and colleges.

In an interview with the Guardian, Collins said he remained convinced the country was underinvesting in education overall, and warned that the evidence emerging in recent months about the impact of lost learning suggested that the NTP, set up with great fanfare to help left-behind children catch up, was not delivering.

“I worry we’re into that ‘Don’t mention the war – don’t mention Covid. Let’s just pretend we can put that behind us. Let’s put our head in the sand.’ And that won’t wash because the impact of this isn’t going to go away unless we do something about it. We’re letting down a generation of children if we don’t do it,” he said.

The delivery of the NTP, which has been led by the Dutch HR giant Randstad this year, has been widely criticised with low participation rates in some areas of the country. As a result, the Department for Education (DfE) announced that all funding would go direct to schools next year so they can arrange their own tuition, a move welcomed by headteachers.

On the risk of tax cuts, promised by various contenders in the Tory party leadership race, Collins warned of “restraint on spending on public services including education”, an area where there is “no fat”.

Speaking of his own time dealing with the Treasury, he said the department saw education spending as something to control, rather than core to long-term investment.

“I was given lots of encouragement and was being told to be ambitious by No 10 people, including the prime minister, but it was at the Treasury where the brakes came in,” he said, calling the thinking a false economy.

“Every piece of evidence indicates that the investment I was asking for was tiny compared to the long-term hit if people lose earning power and economic productivity.”

He added: “I’m incredibly disappointed – and there is an anger – because our children and our schools deserve more You’ve got children who never had any time in reception, or in the early years – and that’s a big part of your life missing. Huge rites of passage are missing – your prom. All missed. And in the long term they matter.”

“Then you’ve got all the children that didn’t sing in a choir for two years, or didn’t take up a musical instrument or play sports. They’re reluctant now to go back to those things, so we’ve seen a drop off in some of those activities.”

Collins had hoped that high-quality, well trained tutors and tutoring, which has been shown to make a difference to outcomes, could become a new arm of the education system, available to everyone, rather than just for the better off.

“But we’ve ended up with tutoring that isn’t as well supported and developed as it should be. There were issues around how the government decided to award the contract for those who were going to do the training and broker the programme and make it available to schools,” he said.

He added: “Unfortunately it hasn’t worked out as well as it should have done. So now the money is going to schools, but are they getting the support they need to develop the tutors? Are they being supported to make sure the tutoring is high quality? I’m not so sure.

The biggest issue in education, however, is the attainment gap between disadvantaged children and their wealthier peers, said Collins. “The gap has been exacerbated and I fear that’s just going to become – well, that’s how the system is, we live with this, it’s nature’s course. And of course that’s unacceptable. I worry about this notion that we can’t do anything about it.”

A Department for Education spokesperson said: “The government has been driving forward work to get children back on track after the pandemic, including through our revolutionary national tutoring programme – with over 1.5 million courses already started – and targeted support for whole areas of the country where standards are weakest.

“From September the programme will be simplified, with all £349m of funding being provided directly to schools, while additional funding to support education recovery in secondary schools will double.”