Some of the smiles looked a little forced when union leaders, employers and ministers linked arms for a staged photo as they signed the school workforce agreement in January 2003.

But the message went out loud and clear: the deal had ushered in a new era of co-operation between these once-warring factions.

In a sense they have been proved right. The “social partnership” that began with the deal has flourished in the past five years and spilled into other areas, such as teachers’ pay and performance management.

It has allowed issues that previously may have resulted in very public rows to be resolved quietly behind closed doors.

But the workforce agreement has also been one of the most divisive developments ever to have hit the teaching profession. It has led directly to the isolation and marginalisation of Britain’s biggest teaching union, nearly lost the backing of the biggest support staff union and split the largest heads’ association down the middle.

Ironically, the Government had been dragged to the table because of unity between the three main classroom unions which threatened action over excessive workload. The then Department for Education and Skills also faced growing teacher shortages. By permitting support staff to teach, albeit with supervision, ministers realised they could ease teacher workload and recruitment in one fell swoop.

For supporters of the agreement, this controversial proposal merely formalised what was taking place in many schools anyway. For the NUT, it compromised the principle that only qualified teachers should teach, so it withdrew before the agreement was signed.

However, the union had a problem: the agreement stood to reduce its members’ workload. That made it difficult for the NUT to back its opposition with industrial action. The only attempt took place at Radclyffe School in Oldham, which had employed four non-teacher “learning managers” to provide cover for absent teachers.

The NUT mounted a high-profile campaign as its members began to refuse to prepare, mark or assess lessons taken by the managers at the comprehensive.

But within a month Hardial Hayer, the head, had called the union’s bluff and introduced a system that forced teachers who took action to cover lessons themselves. Faced with this prospect, local NUT officials decided, without the knowledge of their national leadership, to suspend the action.

Today John Bangs, the union’s head of education, still believes that allowing support staff the bigger role could have damaging consequences that have yet to emerge.

Schools had enjoyed relatively good funding levels since the deal was signed, he said, but future cuts could force them to exploit the agreement by using support staff as cheap teachers.

But he does admit that the NUT might have made a mistake by cutting itself off from negiotiations with the Government, which ruled it out of a “much bigger agenda”.

“In hindsight, we did not realise what the length and breadth of the ending of the relationship was,” Mr Bangs said. “We should have really thought about how we could have changed the agreement.”

The NUT is not the only organisation to have regrets. The leadership of Unison, the biggest support staff union, at times struggled to stay on board. Its members, angered by a lack of progress on pay and conditions, passed conference resolutions against the agreement in successive years.

A parallel battle between a union’s grassroots members and leadership was also raging in the National Association of Head Teachers (NAHT). But in this case the grassroots won, leading the biggest heads’ union to pull out of the agreement in March 2005.

The association had concerns that primary heads - the majority of its members - would not have enough money to introduce planning, preparation and assessment (PPA) time.

Graham Alderton, head of Lambourne Primary in Abridge, Essex, was one of the members backing withdrawal from the deal.

“I believe PPA will be advantageous for primary teachers in the long term,” he said at the time. “However, in the short term it will be a disaster. There comes a time when a stand must be taken.”

But the NAHT’s stand did not last long. In January 2007 it voted to rejoin the partnership, despite not having obtained any guarantee of extra Government funding.

Mr Alderton did manage to introduce PPA time by using a combination of teaching assistants and qualified teachers.

“I suppose people’s goodwill and efforts made things work,” he said. “It wasn’t the big disaster we thought it would be.”