A teaching union stands accused of using a government law it campaigned against to break strike action by its own employees.

Staff at NASUWT, England’s second-largest teaching union, have been blocked from holding further strikes based on a ballot of GMB union members held last year.

Members of the GMB working at the Birmingham-based NASUWT walked out three times last autumn over proposed changes to pensions at the organisation.

We would have expected a badly run academy school trust to make a decision like this but not a sister union

Under the 2016 trade union act, unions wanting to base industrial action on a ballot more than six months old now have to get agreement from the employer. In this instance, the NASUWT refused to allow an extension to nine months.

The decision has prompted dismay among GMB activists, who believe a move to block further strikes in the coming months was based primarily on fears the NASUWT’s annual conference in Belfast over the Easter weekend would be “significantly disrupted”.

The move also prompted accusations of double-standards. The NASUWT not only campaigned against the 2016 act, but has previously used a years-old ballot of members to enact teacher strikes.

According to its 2016 annual accounts, the union “entirely rejected” the proposals for trade union reform in the legislation, claiming they were “completely unnecessary and related to a longstanding ideological opposition to trade unions held by the government”.

“The proposals sought to restrict, in particular, the rights of workers within the public sector. The NASUWT asserted the right to strike as a fundamental right which should be available to all working people, regardless of whether they work in the public or any other sector,” the union continued.

David Warwick, an organiser with the GMB, said the move by the NASUWT was “particularly frustrating” because the union “has been relying on a ballot from 2011, conducted under the previous legislative scheme, in its continuing dispute with the secretary of state for education”.

“GMB members and activists employed by the NASUWT are determining their next steps in this pensions and pay dispute,” he said.

Another unnamed activist added: “We would have expected a badly run academy school trust to make a decision like this but not a sister TUC-affiliated union.”

But Chris Keates, the NASUWT’s general secretary, placed the ball firmly in the GMB’s court, claiming said her organisation “has offered the GMB further talks with a view to resolving the dispute”.

The row between the NASUWT’s leadership and their staff arose last year over proposed changes to pensions paid to staff.

Under the proposals, staff will pay more into their pensions, but get less out once they retire.

In some cases, the amount paid annually to workers will decrease by more than a thousand pounds, documents show.

And last month, further tensions were revealed when the union was ordered to reinstate a sacked employee after an employment tribunal found it was “likely” he was dismissed for whistleblowing.

The NASUWT fired Richard Harris last October after he accused Keates of lying, wasting police time and “acting like a despot” by taking away his company car, a tribunal ruling shows.

The union suspended him in late 2017 amid allegations of gross misconduct. However, he claims Keates wanted to get rid of him because of his activity within the GMB union, and was using the row as a “pretext”.

Paul Housego, an employment judge, ruled that Keates’s actions in reporting Harris to the police and removing his company car “do not seem…likely to be justifiable”, even in the context of a “no-holds-barred political struggle”.