The workers are members of Unison and the Public and Commercial Services (PCS) union. Members were balloted last month after a pay rise offer from Ofsted below average inflation (News, 10 April).
Both unions voted overwhelmingly against the offer, and the strike includes an overtime ban from 19 to 30 May.
Unison general secretary Dave Prentis said, 'They are striking over the appalling treatment being meted out by Ofsted management, who are piling on the work but forcing three-year pay freezes on to some staff. Inflation is running at over 4 per cent and even the Government's preferred measure jumped to 3 per cent this week.'
Mr Prentis added, 'What we need now are some serious proposals to avert further action.'
Mark Serwotka, PCS general secretary, said, 'The Government has got to wake up to the growing unrest across civil and public services and review its draconian policy towards public sector pay which is provoking such anger, as was seen in the joint union action over pay involving the NUT, UCU and PCS on April 24.'
A spokesman for Ofsted said that good contingency plans would keep disruption to inspection and regulation services to a minimum.
He said, 'Ofsted secured a 3.8 per cent settlement from the Treasury every year for three years. This is better than many other parts of the public sector. We have used this to introduce a fairer pay system under which the vast majority of staff are better off. It was disappointing that the trade union side could not reach a collective agreement over pay priorities.'