Two of Britain's biggest teaching unions have voted in favour of a strike over changes to pensions, in the first sign of collective opposition to the coalition's austerity programme. The ballot in favour of a strike by the National Union of Teachers (NUT) and the Association of Teachers and Lecturers (ATL) is expected to close schools across England and Wales this term, affecting millions of children. The civil service union the PCS publishes the result of its own strike ballot on Wednesday. In the NUT's ballot, 92% voted in favour of strike action, with a turnout of 40%. The ATL result was 83% in favour of a strike on a 35% turnout. The strike will affect both state and private schools. Christine Blower, general secretary of the NUT, the largest teachers' union, said: "The government's unnecessary attack on public sector pensions has convinced NUT members that there is no alternative but to support strike action. "It is disgraceful that the government is pressing ahead with its reforms, which will affect teachers' pensions. The government knows that they are affordable. This is a policy which has nothing to do with economics and everything to do with politics." The strike is expected to take place on 30 June. A spokesman for the Department for Education said the government expects to see heads working to keep schools open. The general secretary of the ATL, Mary Bousted, said: "For the average member [the changes] will be £1,500 a year in increased pension contributions. At a time of a two-year pay freeze, its a 3% cut, which has nothing to do with the health of the scheme. It's a tax on pensions to pay for the deficit." ATL, seen as the most moderate teaching union, has never taken national strike action before. The pension reforms, put forward in a government-commissioned report by former Labour minister Lord Hutton, include raising the retirement age for state employees from 60 to 66 by 2020. Final-salary schemes will be scrapped and replaced by career averages, while ministers will get more powers to raise employee contributions. The government says the cost of paying for teachers' pensions is forecast to rise from around £5bn in 2005 to almost £10bn by 2015 as more staff retire and life expectancy increases. The schools minister, Nick Gibb, was heckled and jeered by teachers as he attempted to justify proposed changes to their pensions, at the ATL's annual conference in Liverpool in April. When Gibb told delegates: "I fully understand the strength of feeling on this" and said teachers' pensions remained a priority, he was greeted by shouts of "no, you don't" and "rubbish" – along with calls for evidence of the need for change. Other unions have warned of possible industrial action later in the year unless negotiations over public sector pensions can lead to a deal. The GMB has said it is prepared to ballot its members, including those in local government, if the deadlock is not broken. Last year, the mayor of London, Boris Johnson, urged the government to introduce legislation preventing industrial action unless at least 50% of union members in a workplace take part in a ballot.