The introduction of performance-related pay in schools risks a surge in fraud as teachers falsify data to secure a pay rise, heads' leaders and employment lawyers have warned.
Legal experts also predict a rise in sexual and racial discrimination claims from teachers as a result of the pay reform, which they say will create an "administrative nightmare" for schools.
From September, automatic annual pay rises will end, with schools free to set criteria for awarding increases to teachers. Advice published last week by the Department for Education (DfE) said that schools could tie salaries to a range of measures, including appraisals, test scores, self- assessment, peer review and even the views of students and parents.
Ministers believe the move will help schools to reward and retain the best staff. But lawyers are predicting a rise in formal complaints to governors and in legal challenges and cases of teachers trying to cheat test scores to win a salary increase.
"There is the potential for fraud, particularly if there is lots of classroom-based work. There has to be the potential for that to increase," said Mark Leach, employment partner at law firm Weightmans. "Where you link performance to reward, there is the potential for performance to be over-egged."
Russell Hobby, general secretary of the NAHT heads' union, said that new pay systems based too crudely on data would be "almost asking for trouble".
"With any major change, there will be a rise in people testing the edges of (what is permitted)," he said, adding that schools should use "a range of qualitative and quantitative" measures to limit the potential for fraud.
Last month in the US, 35 school staff were indicted in relation to a testing scandal linked to performance pay. An investigation found that educators in 44 schools in Atlanta, Georgia, had told students the answers to test questions or changed answers after the papers had been handed in. Schools with good test scores received extra funding to spend in the classroom - or on bonuses for teachers.
Brian Lightman, general secretary of the Association of School and College Leaders, said: "Any teachers (behaving fraudulently) would be carrying out an act of gross misconduct and putting their job at risk. It goes without saying that we would strongly advise against any attempts to play the system."
Heather Mitchell, an employment solicitor at Browne Jacobson, said a rise in the number of complaints from teachers was also likely. "Even with consistent, cast-iron policies, there will be lots of people who. feel aggrieved by pay decisions," she said. "It will be a real administrative burden and pressure on governing bodies."
A rise in complaints from teachers would "increase the atmosphere of dispute in schools", she added.
Schools must be seen to be consistent in how they award pay rises, Mr Leach said. "Where that is not seen to be in place, that is when complaints will be made - allegations such as: he only got a rise because he's white, heterosexual or not too old. I can see a real administrative nightmare for schools."
The NUT and NASUWT unions, which oppose the reforms, will begin a programme of national and regional strike action in June. But Christine Blower, general secretary of the NUT, hit back at the idea that teachers would commit fraud. "This is a direct accusation that teachers lack integrity," she said.
Emma Knights, chief executive of the National Governors' Association, warned that tension between the unions and the government could trigger disputes in individual schools.
"We're worried that, with the unions' stance, there will be complaints, even if (they) don't have much substance behind them," she said. "We're introducing this new system at a time when there are already issues with the unions, and that makes the process much more difficult than it needs to be."
A DfE spokeswoman said it was "vital that schools can recruit and reward the best teachers".
"We are reforming pay so schools can attract and retain the best teachers, who have the greatest impact on their pupils' achievements," she added. "Schools will develop their own pay arrangements. Their pay policy must set out in detail how all pay decisions will be made, including how appraisal outcomes are linked to these decisions."