The letter emerged as Downing Street yesterday backed plans for parents to staff classrooms during the walkout, with ministers appearing increasingly determined to face down militant trade unions.
David Cameron will make a last-ditch direct appeal to public sector workers today not to go on strike on Thursday, insisting their current pension arrangements are “not fair to the taxpayer”.
The Prime Minister will address the Local Government Association’s annual conference to warn council workers and teachers that the “situation is unsustainable” and that they must accept changes.
Downing Street sources said that Mr Cameron would be “robust” but would attempt to set out a “fair argument” over why reform of pensions was essential.
"Four unions balloted their members and reached that decision, and that reflects the degree of anger and worry and real fear there is across everyone who works for public sectors that their pensions are under threat," he added.
Up to 750,000 teachers and civil servants will take part in the biggest industrial action since the coalition government came to power in May 2010.
Mark Serwotka, head of the civil servants' Public and Commercial Services (PCS) union, said "not one jot of progress" had been made with the government, which wants public sector workers to pay more into pension schemes and retire later.
Unions are also angered by proposals to inflate pension payouts according to the usually lower consumer prices index (CPI) rather than the retail prices index (RPI).
PCS members will be joined by workers from the National Union of Teachers (NUT), the Association of Teachers and Lecturers (ATL) and the University and College Union in Thursday's strike.
Thousands of schools are expected to be hit by the action.
Cabinet Office Minister Francis Maude argued that the talks had been "constructive".
"This is a genuine consultation to which we are committed in order to try and agree a way forward with the unions, including on how to implement the changes on contributions set out in the spending review,
