The National Union of Teachers revealed this week that its membership has grown by some 5,000, in the six weeks since it announced a ballot for action over living allowances in London and the South-east.

More than 1,500 schools were either partially or fully closed last Thursday in the protest over the “inadequate” rise in the allowance from pound;3,000 to pound;3,105 in central London.

Only around one in four schools across the capital and surrounding areas is believed to have escaped major disruption; an estimated 120,000 children stayed at home.

Around 10,000 teachers expressed their anger in a protest march from Holborn to St Pancras. A rally at Camden town hall ended with a unanimous vote in favour of joint action with Unison and other public-sector unions over the cost-of-living allowance.

Education Secretary Estelle Morris compared the strike to the days of union militancy of the 1980s. Iain Duncan Smith, leader of the Opposition, said it was harmful to children.

But NUT general secretary Doug McAvoy said: “Teachers are only too aware of the damage to children’s education that shortages in the capital are causing.” He added that he could not rule out the possibility of further strike action on the issue.