Teachers who are planning to join Thursday's strike are being intimidated and told they could face disciplinary action if they walk out over pension reforms, a teaching union has claimed.

Mary Bousted, the general secretary of the Association of Teachers and Lecturers – which is striking for the first time in its 127-year history – said her members had reported being threatened with disciplinary action if they strike, or told that their actions would bring their school into disrepute.

One London borough, Conservative-led Hammersmith and Fulham, wrote to staff to tell them to "ignore plans to strike" and that it was advising those intent on taking part in the walkout that they should make up the time to pupils. Helen Binmore, the borough's cabinet member for children's services, said: "We have suggested to the schools that, as the strike is a foreseen closure, they should consider offering their pupils an additional day's education where possible."

The government said that more than 5,300 of England's 23,000 schools were expected to be affected by the strike with nearly 3,300 confirming they will close. The education secretary, Michael Gove, was in talks with individual union leaders by phone in an attempt to avoid strikes.

Bousted said the Hammersmith and Fulham letter was a "deliberate move to intimidate" teachers, and the suggestion that teachers should work an extra day to make up for strike action was "completely inaccurate and wrong".

Gove said that 3,206 state schools in England are confirmed to be closing on Thursday along with 84 academies. A further 2,206 state schools and 128 academies will be partially closed. The rest are unknown but government officials will be updating the figures on Wednesday.