In the latest sign of the increasingly fierce battle being waged between companies for elusive staff, Timeplan and Teaching Personnel are under investigation by the Advertising Standards Authority, after a complaint from one of their rivals, Select Education.
In last week's edition of The TES, Timeplan asserted it was "the UK's largest teacher-recruitment specialist", while Teaching Personnel stated it was "the UK's largest specialist teaching agency".
Meanwhile, three weeks ago, a third firm, Capita, claimed to be "the nation's largest network of supply teachers".
Select chief executive Bob Wicks said he had no quibble with Capita's claim. But he said that Timeplan was only the fifth or sixth largest company, while Teaching Personnel probably ranked fourth. His own company was vying for second place behind Capita.
Tish Seborne, managing director of Timeplan, said the company had the largest turnover of any company whose sole business was supply teaching. Capita and Select had other areas of business. Nonetheless, Timeplan has agreed to change its advert while the matter is investigated.
Ms Seaborne added: "At this time of teacher shortage, I am surprised that Select have nothing better to do than complain about our adverts."
Teaching Personnel is also under fire for claiming to have "more branches covering more areas than any other agency". The company, which admits it operates out of one office in St Albans, has now changed this part of its advert.
But Graham Hellier, chairman, said he had no plans to change the company's statement that it was "the UK's largest teacher recruitment specialist".
* Fiona Eldridge, head of regional recruitment strategies at the Teacher Training Agency, will succeed Mr Hellier as chairman of Teaching Personnel next month.