Headteachers are led to believe the work will be done free. But they are later billed pound;2 for every linear foot of white-painted line, costing them thousands.

The Scambusters partnership, part of the Department for Business, Enterprise and Regulatory Reform, has now warned schools to be on their guard. The alert follows cold-calls by a Lancashire firm to schools in London, Middlesex, Osfordshire, Hertfordshire, Leicestershire and Devon.

Steve Playle, Scambusters’ project manager, said the workmen, who come from the Traveller community, became intimidating if payment was not made. “Only later does it become apparent exactly what the charges are and things can turn very nasty,” he said.

Melanie Aley, headteacher at Thorpe Acre junior school in Loughborough, said two “aggressive and persistent” men repeatedly turned up in a white van until she finally agreed to see them.

Their foreman claimed to be from the council’s highway and transportation division and said he had left-over roading materials with which he had been told to help out schools.

“He got quite belligerent,” Mrs Aley said. “He was thickset and heavily built, quite a physically imposing person and, being a female head, you do feel vulnerable.

“When I insisted on a quote, he realised the game was up. They got back in their van and drove off.”

Next door, at the 135-pupil Thorpe Acre infant school, headteacher Mari Street was not so fortunate. She was billed pound;1,379.45 after letting the men repaint the carpark and two zebra crossings.

The men operate under at least two company names, registered to an office in Oldham, Lancashire. The directors are a couple from Bedworth, Warwickshire.

* If your school is approached, email steve.playle@surreycc.gov.uk