The country's only for-profit exam board will lose a portion of earnings from its #163;23 million contract to administer national tests if a boycott takes place, The TES has learnt.
Officials were so concerned about the prospect of industrial action against this year's key stage 2 Sats that they wrote clauses into the deal with Edexcel to ensure the board would not be paid as much if some tests do not go ahead.
Barring any successful local or central government legal action, a national boycott now looks certain next month.
The National Association of Head Teachers (NAHT) and the NUT confirmed last week that they would proceed with the action, saying they had a mandate following a ballot of their members.
The Qualifications and Curriculum Development Agency (QCDA) would not give any details of how exactly Edexcel's fee could be reduced by a boycott, or by how much.
But a spokesperson for the agency said: "The contract allows for a reduction in cost if reduced volumes of test scripts are submitted for marking."
Between them, the two unions represent the heads of the vast majority of England's 17,000 primary schools and so could potentially cost Edexcel a lot of money.
But turnout among NUT school leaders was only 34 per cent and just under 50 per cent for the NAHT, and only 29 per cent of the leaders eligible to vote said yes to the action.
Some heads have told The TES that they will ignore the boycott. The NAHT has revealed that its own research shows that at least 11,000 schools would not take part in the action, which would be welcome news for Edexcel.
The board stepped in at the last-minute to run the 2009 tests for #163;25 million after ETS had its five-year contract prematurely terminated.
Disorganised training, multiple computer glitches, missing scripts, a failing helpline and delayed results for 1.2 million pupils prompted a crisis under ETS in 2008.
Asked how much Edexcel stood to lose from a boycott this year, Ziggy Liaquat, its managing director, said: "We have a contract with QCDA and contractually both organisations are covered in respect of a boycott and that's all I'm going to say because it's a commercial agreement and I can't comment any further."