The British government came under fresh pressure Thursday over its botched policy for grading exams cancelled during the coronavirus lockdown, as a new set of results showed a surge in top marks.

Schools minister Nick Gibb admitted he was warned weeks ago about the risks of using an algorithm to moderate assessments made by teachers, which the government abandoned this week after an uproar.

He insisted the model, which was intended to protect against potential widespread grade inflation, was fair but said it was implemented incorrectly.

The government said it would revert to teacher assessments after pupils took to the streets in protest at last week's A-level results, sat by 17 and 18-year-olds. 

The "standardisation" process saw 40 percent of results downgraded, with children in disadvantaged areas particularly affected.

But a new set of results published Thursday, for GCSE exams sat by 16-year-olds, seemed to confirm fears that the unmoderated system would lead to grade inflation.

Some 25.9 percent of pupils received top marks -- up from 20.6 percent last year. The pass rate was 76 percent, up from 67 percent.

Gibb apologised to all students for the "pain and anxiety" they have suffered because of the chaos. However, he could not say when another cohort of young people waiting for their Btec vocational qualification would get them, after the results were delayed at the last minute.

Prime Minister Boris Johnson's government has been criticised for many aspects of its response to the coronavirus outbreak, which has killed more than 41,000 people in Britain -- the worst toll in Europe.

But the exams chaos risks being particularly damaging, affecting voters who do not normally follow politics.

While Johnson's Conservatives have led the main opposition Labour party in opinion polls throughout the virus crisis, a new YouGov poll found this slipped from nine points to two in the past week.

Labour education spokeswoman Kate Green said the government was "warned again and again about the problems with the grading algorithm" but did nothing. "This endless pattern of incompetence is no way to run a country," she said.