The union faces a possible six-figure sum in damages plus undisclosed costs in a ruling which is not only a financial blow but also has serious implications for supply teachers.

Mr Justice Evans Lombe also ruled that supply teachers who are given work in schools by agencies are not employed by the local education authority. That means they are not subject to national pay rates and conditions of employment .

Timeplan Education Group places around 750 teachers in schools in London and the South-east every week, many of them from New Zealand and Australia. The agency issued a writ after the union wrote to a New Zealand magazine in an attempt to stop it carrying advertisements for the company.

The agency now has plans for nationwide expansion, forcing other agencies to compete on the same terms and conditions. TimePlan managing director Tish Seabourne said: “We welcome this judgment - it makes sense both for schools and teachers.

“If we had not won this case then agency teachers would have had to pack up their suitcases and gone home.”

She denied that her agency was underpaying teachers, many of whom were from overseas or newly qualified British teachers, saying: “85 per cent of them are actually receiving more than they would if they were being paid according to national guidelines.”

NUT general secretary Doug McAvoy said that the union would intensify its campaign against agencies and would be asking for LEAs’ support. He said: “The union is fearful that if agencies are seen as employers of teachers then teachers will see a consequent worsening of their terms of employment and increased vulnerability.”

Ray Mercer, director of the Capstan teacher agency, based in Greenwich, south-east London, has urged Education Secretary Gillian Shephard to legislate to force private firms to abide by national pay agreements. The ruling “could open the door for agencies used as a ‘Trojan horse’ to undermine teachers’ pay,” he said.