Heads are furious about the Prime Minister’s latest get-tough message to schools that produce low GCSE results, after he warned them that they face the sack.

Gordon Brown used his crucial Labour Party conference speech this week to ratchet up the pressure on hundreds of secondaries that have been targeted by the National Challenge scheme for failing to get 30 per cent of their pupils to achieve five good GCSEs.

The controversial school improvement scheme was supposed to offer support to these schools, with closure used only as a last resort. But Mr Brown’s speech put the emphasis on replacing leadership teams and closing schools. He also said that parents would play a direct role in the process.

“Our pledge today is that any parents whose local state school falls below the expected standard will have the right to see that school transformed under wholly new leadership, or closed and new school places provided,” he said.

The TES understands the Government’s social partner unions were warned that the rhetoric would be toughened up, although official policy remains the same.

Mike Lambert, head of The Wordsley School in Dudley, West Midlands, where 27 per cent of pupils achieved five A*-C grade GCSEs including English and maths this year, said the level of criticism was “criminal”.

“People who work in schools are committed to improving them, and to keep hammering them is absolutely ridiculous,” he said.

“National Challenge makes it more difficult for those schools to get good teachers because of the pressure they will be put under.”

Mr Lambert said parents already voted with their feet if they were unhappy with school performance.

John Dunford, general secretary of the Association of School and College Leaders, said: “Schools have never been turned around by threats. National Challenge schools need a period of sustained support and confidence building.

“It was the Prime Minister, not Ed Balls, who originally described these schools as failing, and I think this will be interpreted as being directly led from Downing Street in a way that is wholly inconsistent with promises of support from Sanctuary Buildings (the Department for Children, Schools and Families).”

Mr Balls, the Schools Secretary, used his speech to the conference in Manchester to emphasise the positive aspects of the National Challenge scheme.

He revealed the first three National Challenge Trusts, which will each get up to Pounds 1 million to help schools collaborate to improve standards. They will be in Southend in Essex, North Yorkshire and Hull. Lancashire county council is also considering a trust for schools in Burnley.

Mr Balls announced laws - first revealed by The TES earlier this month - to clamp down on schools that flout the workforce agreement, which is aimed at reducing teacher workload. Schools refusing to honour the deal could lose control of their budgets and have their governing bodies replaced.

He also announced the trialling of universal free school meals for primary children in two deprived areas of England.

And he revealed that the national negotiating body for a school support staff pay and conditions framework, first announced last year, would gain statutory status.

Christina McAnea, Unison’s head of education, welcomed the news, saying this would give the body more teeth in foundation and voluntary aided schools.

But Mr Balls faced criticism from conference delegates over the Government’s attempts to restrict public sector pay rises and its use of the private sector in education.

The Prime Minister unveiled a Pounds 300m plan - also previously revealed in The TES - to help more than a million extra families get online by funding free broadband access and computers.

He also said that any primary pupil falling behind in the three Rs would have a right to personal catch-up tuition.

The Government had already attempted to gain positive headlines from the scheme with a previous announcement of Pounds 315m funding from the Treasury.

But it has attracted criticism from teachers commenting on the TES Connect website, who said the money would be better spent supporting existing literacy strategies.