After months of criticism over its response to coronavirus, the British government is facing a new battle -- from students in revolt over the grading of cancelled exams.

Pupils have taken to the streets and threatened legal action over the decision to downgrade around 280,000 A-level results obtained by 17- and 18-year-olds in England.

The main opposition Labour party has demanded a re-think, and even some members of Prime Minister Boris Johnson's Conservatives have condemned the "shambles".

These were then modified using an algorithm based on a school's past performance, in order to prevent widespread grade inflation.

But critics say the process penalised bright students from disadvantaged backgrounds, while benefiting private school pupils.

More than 250,000 people have signed a petition demanding a change, and two legal groups representing students are threatening to take the government to court.

The director of one of them, Good Law Project's Jolyon Maugham, said pupils had missed out on places at university, medical school and employer training, which all rely on final A-level grades.

"It's also affecting those students who are leaving school to enter the jobs market, the most difficult jobs market in the UK for many generations," he told AFP.

He added: "That's desperately, desperately unfair."

- Issue 'cuts through' -

Britain has been badly hit by coronavirus, recording the highest death toll in Europe -- more than 41,000 -- and the largest second-quarter economic contraction among the so-called Group of Seven major industrial democracies.

Johnson's Conservatives continue to lead opinion polls, but Labour leader Keir Starmer has edged ahead of the prime minister in personal popularity.

The government has shrugged off much of the criticism over its response to the pandemic, from the late imposition of a lockdown to a lack of testing and protective equipment.

But the exam chaos has made many Conservative MPs anxious as angry students and parents bombard them with complaints.

"This is not just one of these bubble issues. This is something that cuts through everything," former Scottish Tory leader Ruth Davidson told Times Radio.

Starmer has sought to capitalise on the row, demanding Johnson interrupt his holiday in Scotland to take personal charge.

"The Tories' chaotic and incompetent handling of this year's exams is robbing a generation of their future," the Labour leader said.

- U-turn looming? -

Many of the government's critics question why it did not anticipate the problem.

The devolved Scottish government had to abandon its policy of grade moderation earlier this month following an outcry, restoring initial teacher assessments for around 75,000 pupils.

Following that U-turn, London said English students unhappy with their A-level grades could appeal on the basis of their preparatory mock results or sit new tests in the autumn.

But just hours after issuing guidance for appeals at the weekend, the exams regulator retracted it -- sparking speculation that a change in policy was imminent.

The row only threatens to get worse as hundreds of thousands of pupils aged 15 and 16 get their GCSE exam results on Thursday.

"We continue to work to come up with the fairest system possible for pupils," Johnson's spokesman said, acknowledging it had been an "incredibly difficult year".

Despite the high percentage of results downgraded, the number of pupils in England, Wales and Northern Ireland receiving the highest A-level grades increased. AFP