Knock-on effects from Anna McGuinness’s victory over her local authority in the Court of Session could force councils to upgrade temporary teachers on to permanent contracts at the expense of existing probationers, who will be expecting to join the jobs scramble over the summer.

Estimates suggest several thousand temporary teachers may be affected by the landmark ruling, which is backdated five years. Any temporary teachers - over all or some of that period - who feel they should have been on a permanent contract will be eligible to make a retrospective claim against their authority. Staffing costs will rise.

Mrs McGuinness has been on annual contracts at Cleveden Secondary since 1994 and three years ago challenged Glasgow to give her permanent status after working continuously over the period. At various stages she has been full-time and part-time but never less than 0.6 of the week. Interest in classical studies varies year to year.

She raised an action through the National Association of Schoolmasters Union of Women Teachers under section 8.5 of the old Yellow Book, which sets out conditions for transfer of teachers from temporary to permanent status. Teachers have to work continuously for a year to qualify.

Pat O’Donnell, the union’s Scottish official, said: “All those teachers who are on a year or more of temporary contracts will be in a strong position to apply to be made permanent. This will lead to increases in the staffing budgets and is likely to have a knock-on effect on probationers.

“It may mean that fewer posts are available for them. It will also put a rocket under the post-McCrone working party on temporary contracts.”

Victor Topping, union president, said: “Schools have been quite happy for the last few years to have supply staff on point 6 or point 7 contracts from the start of term through to the end of June. This verdict will make a total mess of the staffing exercise for teachers coming out in June.”

Ronnie Smith, Educational Institute of Scotland general secretary, said the case reinforced an employment tribunal verdict three years ago. “This is an absolutely crisp, clear, unambiguous and not in the least bit surprising judgment. I hope authorities will learn their lesson and stop abusing people on temporary contracts.”

In his Court of Session judgment, Gordon Coutts, QC, sitting as a temporary judge, concludes: “The situation of temporary teachers could and should have been regulated long ago.”

Mr Coutts stated: “It can well be argued that by operating the present system of employing temporary teachers (Glasgow) are seeking to make economies to the disadvantage of persons who are in fact full-time employees who are called ‘temporary’ and lack security.”

The city may seek to delay the inevitable by appealing against the court’s judgment.