The NASUWT is set to announce this week that it represents more teachers working in England and Wales's state schools than any other union, staking its claim to the position held by the National Union of Teachers for more than a century.
But the NUT has firmly rejected the claim, saying that the official figures prove it remains around a fifth bigger than its rival.
Disagreements between the two unions over membership figures have grown more heated as the gap between them has narrowed.
Chris Keates, general secretary of the NASUWT, said her rival's recent failed strike ballot had given her union the evidence it needed to show it had overtaken.
Last month, the NUT balloted all members working in state schools in England and Wales who could have taken industrial action over pay - 190,269 teachers in total. But the NASUWT said that, had it carried out the same vote with exactly the same restrictions, it would have balloted 202,504 teachers.
Both unions said their figures only included teachers working in state schools. Neither figure included teachers working in further or higher education, local authorities, academies, private schools, on supply, or who have retired.
Ms Keates added that her union would have had even higher numbers - reaching 217,460 - if it had included all its serving members in UK state schools; unlike the NUT, it also has members in Northern Ireland and Scotland.
"This is the first time it has been possible to compare like with like over membership," she said. "The NUT has always claimed that the NASUWT figures only looked larger because we covered the whole UK while they only organise in England and Wales. It is now clear that we are not only the largest in England and Wales, we are by far the largest in the whole UK."
Ms Keates said she felt the shift in popularity had started in 1998, when her union had engaged with the Government over changes to pay, including the introduction of the threshold. Its willingness to co-operate through the workload negotiations was also important. The one-day strike this year had also helped it win members.
The union has seen its membership grow faster than its rival recently, increasing by 8 per cent - compared with the NUT's 3 per cent - over the past two years.
But the NUT poured scorn on the suggestion that it was no longer the largest union, and questioned the validity of its rival's claims.
John Dixon, assistant secretary with responsibility for membership, said its ballot figures had not included advisory teachers or supply staff.
And the NUT had a clear lead in the official figures that unions file each year to the certification officer. "To make the extrapolations they have from the ballot figures is laughable, frankly," he said. "The figures that matter are the official ones."
Mr Dixon pointed out that the NASUWT had tried various tactics in the past to make it seem larger, such as calling itself "the largest UK-wide teachers union" - a claim that was technically correct because the NUT does not have members in Northern Ireland, even though it had more members overall.
John Howson, an education recruitment analyst, said it was quite possible that the NASUWT had taken the lead because it tended to have more secondary members, fewer of whom would have been affected by the dip in primary pupil numbers. But the balance could shift back, he said, when the dip shifted to secondary schools and primary numbers picked up.
However one interprets the figures, the Association of Teachers and Lecturers remains in third place. Mary Bousted, its general secretary, said: "When it comes to supporting members, it's not about who's the biggest, it's about who's the best."
MORE THAN A CENTURY OF UNION RIVALRY
1870: The National Union of Elementary Teachers is founded, to represent all school teachers in England and Wales
1889: The NUET becomes the National Union of Teachers
1906: The London section of the National Federation of Women Teachers leaves the NUT 1909 The NFWT joins the Women Teachers' Franchise Union to form the Union of Women Teachers (UWT)
1915: The NUT moves its headquarters to Hamilton House, London WC1, where it remains to this day
1919: The National Association of Men Teachers is formed within the NUT to promote the interests of male staff. It is set up in response to the union's decision to back equal pay while many male teachers are still in the army after the First World War
1920: The NAMT changes its name to the National Association of Schoolmasters (NAS)
1922: A vote at annual conference bars NAS staff from remaining members of NUT
1945: The National Union of Uncertified Teachers, set up in 1913, joins the NUT
1970: The non-striking Professional Association of Teachers is formed, becoming Voice in 2008
1976: The NAS and UWT merge to form the NASUWT, partly as a consequence of the previous year's Sex Discrimination Act. They are also joined by the Scottish Schoolmasters Association, set up in 1933
1978: The Assistant Masters Association, founded in 1910, and the Association of Assistant Mistresses, merge to form the Assistant Masters and Mistresses Association
1984: Both NUT and NASUWT begin rolling industrial action running until 1986
1993: The Assistant Masters and Mistresses Association becomes the Association of Teachers and Lecturers. It goes on to gain members from the NUT and NASUWT
2003: The NUT becomes the first teachers' union to refuse to sign the workload agreement, creating a rift between it and those in the "social partnership", which includes the Government, NASUWT and ATL
2008: The NUT holds a one-day national strike over pay, then decides against further action
How do you quantify membership of professional associations?
The latest calculations from the NASUWT teachers' union show them with a membership growth of around 80,000 over the past decade.
They also claim more teaching members than the National Union of Teachers, traditionally the largest of the associations.
But perhaps that's not surprising at this point in history, because the NUT has been traditionally stronger in primary schools - where pupil rolls are falling - while its rival has always garnered more support in secondary schools.
Clearly, the NASUWT has been gaining members from somewhere, and full-time teacher numbers in secondary schools are up by 11,000 over the past decade, whereas they are down by nearly 10,000 in the primary sector*.
Nevertheless, the number of teachers contributing to their general fund - effectively paying a subscription - was higher in the NUT by around 27,000 at the last count.
Over the next decade, teacher numbers are likely to fall in the secondary sector and rise in primaries as demographic trends reverse direction, offering threats and opportunities for all associations.
So, how can one account for the apparent difference between the figures from each organisation?
Teachers in local authority service, the private sector, working as supply teachers, and in further and higher education, as well as special schools, account for some members not included in the comparison made by the NASUWT.
There are also student and retired members, but the former don't pay a subscription and may join more than one association.
There are also teachers in the devolved regions and small numbers in the Channel Islands, Isle of Man, and working in forces schools.
Reaching an effective number is never easy, but any outside observer must wonder why there are still so many different associations? Variety may be the spice of life, but it isn't always helpful in negotiations with government and employers.
* DCSF Workforce Statistics 2008 http:www.dcsf.gov.ukrsgatewayDBSFRs000813SFR262008_tables.xls Tables 1 amp; 2
John Howson is a recruitment consultant and visiting professor of education at Oxford Brookes University.