Britain is about to face one of its largest walkouts of public sector staff, the GMB union has said, after as many as two millions workers agreed to go on strike.

The trade union has announced that its members in councils and schools, which make up part of its 620,000-strong membership, have voted three-to-one to stage a mass walkout over pay and conditions on 10 July.

A release by the union said it "looks like being the second biggest dispute ever," once added to the numbers of at least two further unions also staging action.

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The 24hr action across England, Wales and Northern Ireland, is in response to workers “serving our community” who are “ignored and undervalued,” the union said, experiencing a pay hike of just one per cent since 2010.

“In October even the national minimum wage will overtake local authority pay scales,” said GMB National Secretary Brian Strutton. “Our members have spoken loud and clear and said they want to go on strike.”

The action coincides with that of Unison, another trade union, whose local government and school workers have also voted for industrial action for the same day.

“We’re expecting 500,000 to go on strike,” a spokesman of Unison told The Independent. “There will be demonstrations organised across the country at branch level. We’re certainly campaigning to get everyone out.”

The National Union of Teachers (NUT) also decided, at its meeting on 19 June, to call a day of industrial action for 10 July for pay policies which it says are “leading to a teacher recruitment crisis that will be very damaging to our young people.”

It said that 250,000 of its members are affected by the pensions dispute, in which teachers have been asked to pay more towards their pensions which many “cannot access in full until they reach 68.”

A fourth union, Unite, will be closing its ballot for strike action on Monday morning with a result expected that afternoon – possibly increasing the number by a further 70,000.

A Unite spokesman said: “All the indications leading up to the ballot are that people are angry, people have seen their pay drop in real terms by 20 per cent since the Coalition’s come to power. They are saying enough is enough.

“The important thing to realise is that these people are the lowest paid in the sector. They do all the things we take for granted – sweep the streets, look after our kids. It’s a last resort.”

When asked how many people across the entire workforce and four unions could walk out, an NUT spokesman said: “Two million sounds about right.”