The National Union of Teachers could take further strike action after Easter, its leaders have announced, as a leaked memo revealed significant cracks were emerging in its relationship with the NASUWT.

Members of the National Union of Teachers (NUT) will be taking part in a one-day strike tomorrow over pay, pensions and working conditions, with thousands of schools across the country expected to be affected. The NASUWT, which last year embarked on a joint programme of industrial action with the NUT culminating in a series of regional strikes, is not taking part.

Kevin Courtney, the NUT’s deputy general secretary, told TES that the union would decide the next phase of industrial action at its annual conference in Brighton next month, and said there was still a possibility that the NASUWT could be involved in future action.

The NUT is also focusing its efforts on raising public awareness of the reasons behind its campaign, with teachers manning 150 stalls around the country over last weekend to explain why they are taking action.

However a leaked internal email apparently sent out by NASUWT general secretary Chris Keates to the union’s regional secretaries suggests that, behind the scenes, tensions between the two traditional rivals could threaten the union’s historic joint declaration made two years ago that they would work more closely together.

The email criticises the NUT’s “abusive social media campaigns” and “aggressive accusations”, as well as alleging “threats, insults [and] attempts to intimidate our members”.

The message, which has been posted on social media websites, informs NASUWT members that, while they should not cover for striking NUT colleagues, it is “not the responsibility of the NASUWT and its members to make the NUT action successful”.

It continues: “We should not tolerate any threats, insults or attempts to intimidate our members or activists by the NUT. Unfortunately, in some areas, this has been a hallmark of the activity to date. The NASUWT, as an independent trade union, has made its decision with regard to industrial action strategy and that should be respected by a sister trade union.

“You will note that there has been no public criticism by us of the NUT despite the abusive social media campaigns being run by NUT and the aggressive accusations made against us nationally which I am sure your National Executive Members will have briefed you about. Members must be supported robustly and protected if they face pressure or abuse from the NUT and any incidents should be reported to your National Executive Member and copied to me.”

The memo also claims that the NUT ignored the NASUWT’s requests to wait until the publication of the School Teacher Review Body’s (STRB) report before announcing its strike plans.

Going on strike at this stage, the email says, “would unnecessarily alienate the public and parents and provide a significant propaganda opportunity for the Secretary of State”, it adds.

In a recent blog, NUT executive member Martin Powell-Davies accuses the NASUWT of “playing membership games” by responding to the NUT’s strike announcement by announcing a “free membership offer”. The NUT, he adds, had now done the same. “So, far from building genuine unity, it seems that, for now, the long-standing battle for membership between the NASUWT and NUT will continue,” he writes.

An NUT spokeswoman denied that the union had made any public criticism of the NASUWT. The NASUWT declined to comment on the leaked email.

Kevin Courtney also told TES that ongoing talks between the unions and Mr Gove, which have been suspended until after Easter, “haven’t led to an improvement in teachers’ working lives.” He said the union “remains open” to the possibility of taking further joint action with the NASUWT.

The NUT’s national executive will meet next week to decide what proposals for industrial action should be put before its annual conference.

In a letter by Mr Gove published today following the initial series of meetings between the unions and Department for Education officials, he proposes publishing additional advice for schools on how to implement performance-related pay for teachers. The advice, he suggests, should stress “the expectation that schools operate a ‘no surprises’ policy” for making pay changes, and “the need for schools to balance the requirement for robust evidence with the need for proportionality”.

Mr Gove also suggests working with the unions to draw up a joint study into the “health and deployment” implications of the retirement age being put back to 68.

In response, NUT general secretary Christine Blower said Mr Gove, who had not personally attended any of the meetings, had ignored many of the unions’ suggestions. “His letter confirms why we are right to strike,” she added.