Teachers at a London free school faced down intimidation from bosses yesterday to launch a two-day strike over recognition of the National Union of Teachers (NUT).

One of a dozen on the picket line outside the Stem6 school in Islington held a sign reading: "What a shameful way to treat teachers."

The action was called on Monday evening after principal John O'Shea reneged on an agreement to recognise the NUT.

Staff have also revealed how management told them to sign zero-hour contracts or "face legal consequences" and forced them to attend intimidating meetings without union representation.

After refusing to attend alone, teachers agreed to go in pairs but found themselves outnumbered by three bosses.

Local NUT secretary Ken Muller told the Star: "That's why the teachers want to be in a union.

"They are more determined now than they were when the dispute stated several weeks ago.

"All of our members were on strike and morale was high."

A strike was called off last Wednesday after Mr O'Shea promised to sign a recognition agreement with the NUT.

In an email, he wrote: "Just to confirm that we are willing to recognise the NUT and a commitment to enter into meaningful negotiations about the terms and conditions about the contract and to install a local and national representative within the academy."

But he postponed a meeting with union reps on Monday and made a U-turn on his pledge.

When determined staff announced the latest walkout he said the school would be closed due to the Tube strike.

The teacher's strike will continue today and across three days next week unless bosses offer a new recognition deal.

Mr O'Shea has already been forced to drop zero-hours contracts but teachers are calling for improved pay, sick pay and maternity leave.

Mr Muller said: "They did make some concessions. We don't think that would have happened if we hadn't taken a firm stand over it."

NUT deputy general secretary Kevin Courtney joined his members on their picket line alongside other local teachers and trade unionists.