Boris Johnson’s former schools catch-up czar has launched a withering attack on the prime minister’s “feeble” response to the pandemic and warned children cannot wait until the autumn for a £15bn cash boost he demanded.

Sir Kevan Collins revealed that he had warned both Johnson and the chancellor that without a big increase in funding for extra school time, the loss of learning from Covid would hit pupils’ life chances and trigger a huge hit to the UK economy over the long term.

The former education recovery commissioner, who this month quit in protest at being given just £1.4bn for his programme, confirmed that it was a tenth of the cash he had called for.

He called for a 10-year spending plan, saying it should be “guaranteed” in order to ensure inequalities in education that existed before the pandemic were not worsened even further.

In a major blow to the PM, Collins resigned after the Treasury reportedly objected that there was insufficient evidence that his plan to add half an hour to the school day would be workable.

Speaking to the Commons education committee, he hit back hard, arguing that his plan’s cost was small compared to the economy of between £100bn and £420bn in lost productivity and skills.

And he said pupils couldn’t wait until the Treasury’s November spending review, arguing pupils needed extra classroom time from when term starts in September to make up for social, emotional and academic skills.

“Our country’s responded in a way, compared to some others, which quite frankly is a bit feeble,” he told the MPs.

“Because the amount of money we’re responding with at the moment, these are significant sums but this scale of shock, losing on average 115 days of face-to-face learning, requires a massive national effort to recover.”

Sir Kevan said he was “very worried” about “complacency” and the belief that children’s education would recover naturally.

He pointed to the Netherlands, where pupils have been given an extra £2,000 each to catch up, compared to England’s planned £50 per head.

Explaining his decision to quit, he said: “The quantum was so different from the amount that I thought we needed to deliver the exam question I was asked that it was impossible for me not to step back at that point.

“I think the recovery definitely will take a number of years, I don’t think this is a quick-fix catch up. A 10-year plan would be for me about guaranteeing that funding over the long period,” he said.

The prime minister’s official spokesperson denied the sums allocated so far were “feeble”.

“I think the public would accept that £1.4 billion is a significant sum of money, and comes up on top of an additional £1.7 billion,” they said.

“We will also consider what further funding is necessary in the SR [Spending Review].”

Collins – who said he had personally made his case to Johnson, Rishi Sunak and Cabinet Office minister Michael Gove – accepted that more money was expected later this year but insisted that the Treasury had to accept the urgency of the crisis.

“We can’t wait. I know there are these processes known as the spending review that fit within a government cycle, but I’m more obsessed with this education cycle.

“And we need to start from September. I wanted to hit the ground running this September because children can’t afford another year.

“Everybody accepts that there is a link between the economic wellbeing of the nation, and the productivity in education terms, the human capital investment. I think a key question might be: what is the Treasury’s own model of the loss?

“We cannot blight a generation of children by not investing in our education. I was ready to be known by the end as the Education Recovery Commissioner that might have suggested we were spending too much on English children.

“I’d rather be that side of this wrong than the side of wrong which is we have underestimated and under invested.”

He said his big worry was pupils leaving primary school without sufficient reading or maths skills.

“If children are struggling with their reading in Year 7, the textbooks get harder, you get more subjects, and you don’t catch up. In fact you build a compensatory behaviour to reading and it actually can get worse. The children can’t wait so I wanted to break out of the spending review cycle.”

And he added the poorest children were being hit hardest.

“I’m personally very, very clear that the biggest impact of Covid will definitely be on our most disadvantaged children. The growing education inequality could be the legacy of Covid if we’re not very careful,” he said.

The former czar said that while tutoring programmes were useful if done well, there was a need for more classroom time with teachers and fellow pupils.

“It’s not a bit of tutoring in the corner, it’s actually a fundamental approach. That’s why I was keen to see a whole-school effort around time, around teaching, around tutoring, and not a narrow kind of auxiliary attention to one particular activity that gets left to a teaching assistant at the back of the class.”

Critics have complained that when Johnson appointed Collins he appeared to have his full backing but then allowed his plans to be watered down.

Collins told the MPs: “Of course I presented to the prime minister, to the secretary of state and to the chancellor.”