The Scottish Secondary Teachers’ Association (SSTA) insists it will be business as usual at its annual congress next week - despite having sacked its general secretary after only 21 days in the job.

Sheila Mechan’s departure would make “absolutely no difference” to proceedings, said acting general secretary Alan McKenzie, as it was simply “an internal matter involving someone’s employment”.

Ms Mechan started the job on 7 April but was suspended on 19 April. Her contract was formally terminated on Monday. This prompted a war of words, with Mechan accusing the SSTA of being “obsessed” with the EIS teaching union to the detriment of its own members.

A spokesman for the SSTA, which represents about 9,000 teachers, said the decision was “regrettable” but that there had been an “immediate breakdown of a significant number of crucial working relationships” within the union following Ms Mechan’s assumption of the role.

“I am confident that the vetting we did was robust,” said Mr McKenzie, who acted as general secretary for 18 months before Ms Mechan took up the post. “The candidate we appointed was the problem, not the process.”

The union was also right to take prompt action when things started to go wrong, he said, adding that sometimes “early action is better than later action”. Mr McKenzie insisted that neither the union’s credibility nor its morale had been damaged. He added that it would have no problems in attracting a suitable candidate for the vacant post but that the union would not be rushed into an appointment. “We will take as long as it takes,” he said.

The general secretary’s speech to congress sets the tone for the two-day event. This year’s address will now be delivered by Mr McKenzie next Friday. He told TESS that he would cover much the same ground as Ms Mechan would have done and that he would refer to her departure only in a private session.

Ms Mechan, who does not have a teaching background but was an employment law specialist who had worked for the National Union of Teachers in London, told TESS that she was planning legal action. “If I had to sum up my biggest disappointment with the SSTA, it was the union’s failure and lack of focus in terms of serving its membership and supporting school representatives,” she said.

“There was also an issue around a discussion thread on the SSTA website, which was critical of the union. I tried to respond to it, but a decision was taken to remove the thread and even take the whole website down for a time. So much for freedom of speech.”

Ms Mechan said she also had concerns around the training of district secretaries and school representatives, efforts to recruit student teachers, lack of activity on the ground and an “unhealthy obsession” with the activity of the EIS, which she claimed distracted from the SSTA’s responsibilities to its members.

Mr McKenzie rejected all these claims, saying: “That may well be her perception, but that certainly isn’t the reality.”

Sources close to Ms Mechan said that she was only a day into the job when she sensed that problems were arising. They suggested that union colleagues responded in a “resentful and spiteful” manner when she tried to intervene after SSTA members raised complaints about the union in the online discussion thread, and resisted her attempts to draw up a five-year strategic plan for the union.

SSTA sources said they began to have concerns about Ms Mechan when, in an interview with TESS before she started in the post, she took aim at the Scottish Qualifications Authority, describing it as “cloth-eared”. Members had raised concerns that such language was inappropriate and less measured than expected of the general secretary, the sources said.

Ms Mechan, 55, was born in Glasgow and went to Notre Dame High School. She became the first female president of the student union at Elmwood College in Fife while studying business in the 1970s. She launched magazines for Asda and Waitrose during a publishing career, before starting a law degree in 2002.

In February, when TESS revealed that Ms Mechan was poised to become the SSTA’s new general secretary, Mr McKenzie praised her “ebullient personality and natural leadership qualities”, adding: “I particularly like that she has got a good sense of humour but is also quite steely in her approach to negotiating.”