his time last year, as he scurried from one teacher union conference to another, Hank Roberts, the leading campaigner for a single teaching union, felt like a man whose time had come.

A TES poll had shown that a majority of teachers in each of the three main unions supported a merger. The old guard of union general secretaries, often blamed for preventing progress, was on its way out, with the National Association of Schoolmasters Union of Women Teachers and its new boss Eamonn O’Kane appearing ready to embrace unity. And the unions were collaborating more closely than ever before in an effort to force government action over teachers’ workload.

But if a week is a long time in politics, a year represents an eternity in the fractious world of the teacher unions.

Almost immediately the picture became clouded.

Delegates at the NASUWT conference proved less open to the idea of merger than their new leader, and Mr O’Kane suffered an embarrassing defeat.

Peter Smith, the then Association of Teachers and Lecturers’ general secretary, who led his union into the Trades Union Congress, and who viewed unity as an issue which needed to be tackled - “it’s like toothpaste out of the tube, it cannot be put back” - stood down due to ill health.

By the end of the year, cracks had appeared in the unions’ united front on workload, and when the time came to agree a deal in January, it had disintegrated. The National Union of Teachers not only chose to go it alone in opposition to the deal, but placed a number of advertisements criticising the agreement which did nothing to endear it to colleagues in other unions attempting to sell its merits to their members.

“Momentum has been lost,” admitted Mr Roberts, who is a member of each of the three largest classroom unions.

But he was quick to point out that powerful arguments in favour of merger have not gone away. A single organisation could do much to reduce the workload of teachers performing local union duties.

And the prospect of a single teaching union, speaking for more than half a million members, is one which could give ministers sleepless nights.

Senior Tories closely associated with the reforms introduced in the 1990s believe that grant-maintained schools, tests and league tables might all have been blocked had teachers opposed them with a single voice.

“We used to joke that if you put eight teachers in a room, the first thing they would do is form six different unions,” said one.

It is these arguments - and perhaps a wish to exert his authority over his union - that have prompted Mr O’Kane to try again to win support for a merger.

This time around his approach is subtly different. Instead of seeking outright backing for the policy change, unity is only one of a number of options offered for discussion - a move forced on him by the anti-merger majority on his national executive.

If Mr O’Kane wins delegates’ backing and members favour unity, then proposals could be put to a special conference around the end of the year.

The picture in the ATL is less clear, although the surprisingly strong showing of Mr Roberts in the union’s recent election for general secretary will embolden advocates of merger.

With little support from the present leadership, Mr Roberts picked up 40 per cent of the votes cast and came within 104 votes of defeating the executive’s preferred candidate and becoming the union’s new leader.

Opponents will point to a turnout of just 14 per cent and the fact that the vote was split between three candidates.

Mary Bousted, the union’s ambitious new leader, appeared to hedge her bets in her first interview with The TES.

Unity “may be out of the tube but it’s not spreading,” she said.

Both the ATL and the NASUWT have strong reasons to stay independent. As smaller organisations than the NUT, many members fear their identity would be lost in a merger.

Many members and activists of the traditionally moderate ATL have more sympathy with ministers than with the more hardline elements in the NUT.

NASUWT activists tend to view the ATL as a soft touch while looking on in horror as the NUT’s broad left and hard left factions battle for control of union policy.

Many in each union also argue that having more than one voice representing teachers ensures that a range of opinions are heard.

Arthur Jarman, NUT assistant secretary with responsibility for unity, believes that among the biggest obstacles to a merger is the reluctance of local officials in the NASUWT and ATL to cede power - a phenomenon, he says, that helped block the planned merger of the two largest teaching unions in the United States.

“It is a case of turkeys not voting for Christmas,” he said.

Ironically, though, it is his union - the only one which officially supports unity - which has created the most immediate obstacle. Delegates meeting in Harrogate this weekend may pay lip service to unity, but the NUT’s decision to reject the workload deal has left the union isolated.

Senior figures in the other unions suspect the NUT of trying to win membership by taking a populist, but ultimately futile, anti-agreement stance.

Local NUT officers report an upsurge in membership.

“In Calderdale we have seen our membership increase, particularly among supply teachers, as people have left other unions over the workload agreement,” said Sue McMahon, the local NUT official.

She estimates membership has increased by 10 per cent since the deal was signed. “Locally we still work together on issues because we have to, but there is no doubt the workload agreement has driven a wedge between the unions at a grassroots level,” she added.

With signs that the NUT leadership is looking for a way in from the cold, other union leaders are determined the NUT will not be given concessions that would allow it to claim victory at their expense.

As one put it: “I do not think the NUT likes being out in the cold. They are not just out in the cold with the Government, but with the other signatories to the agreement, as well.

“What this illustrates is the difficulty in finding a single voice for teachers. The TUC worked hard to keep everyone on board. If the TUC cannot keep the NASUWT and NUT together what hope does anyone else have?”

Perhaps unsurprisingly, the NUT sees thing differently.

“The fact that there are different views is not a reason to have different unions, it is a reason to have just one which can resolve differences democratically,” said Mr Jarman.

“Who knows whether a single union would have signed up to the agreement?”

Nevertheless, a breakthrough seems unlikely while the unions continue to squabble over such a central issue.

With the NUT membership solidly behind the union’s stance, a real chance for rapprochement may not come until current general secretary Doug McAvoy steps down next year.

Writing in this week’s TES, Steve Sinnott, NUT deputy general secretary and leading contender for the succession, argues that unity is the key to winning workload reduction.

Mr Roberts remains cautiously optimistic. “It is sometimes important to take a longer-term historical perspective - though not necessarily this long.

“The number of British trade unions fell from 1,103 at the end of 1892 to 333 in 1987, and the downward trend has continued since and for very good reasons. Teacher unions will not forever defy this trend,” he said.