The National Association of Schoolmasters Union of Women Teachers says its success in negotiating the workload agreement has helped it to close the membership gap on its larger rival.
Figures submitted to the Trades Union Congress this summer show that the NASUWT now has only 16,310 fewer members than the National Union of Teachers.
Both unions increased membership, with the NUT's going up from 232,280 in 2003 to 239,796. The NASUWT enjoyed a bigger rise, from 211,779 to 223,486.
The gap between the two unions has nearly halved since 1995 when the NASUWT had 29,057 fewer members, according to TUC figures.
But there was bad news for the Association of Teachers and Lecturers: its membership fell from 110,083 in 2003 to 108,730.
The NASUWT says its success is due to teachers' approval of its "social partnership" with the Government, employers and other unions, which has led to three separate agreements over teacher workload and pay.
But the NUT, the only major teaching union not to have signed up to the three agreements, claims much of the NASUWT's increase comes from a recruitment drive in Northern Ireland where the workforce agreement does not apply.
The NUT this week continued its attack on the deal between the other unions and Government on management allowance payments. It warned teachers could lose payments for pastoral work under the proposed new system.
The deal, announced last month, proposes replacing management allowances with "teaching and learning responsibility payments". But the deal makes no mention of pastoral work and, in a submission to the School Teachers' Review Body, the NUT warns it could be in danger. The union wants responsibilities for pupils' personal and social development to attract payments too.
As head of Year 10 at George Green's school, east London, Nigel Orton's duties include monitoring attendance and behaviour. He said: "I use support staff to assist me but it would be difficult for them to have the view of the broader picture that only a qualified teacher does," he said.
Chris Keates, acting general secretary of the National Association of Schoolmasters Union of Women Teachers, said responsibility for general pupil welfare, traditionally given to heads of year, had now passed in most schools to qualified counsellors.
But Chris Watkins, national executive member of the National Association of Pastoral Care in Education, said: "This is all part of turning teachers into 'deliverers' and considering anything else such as pastoral responsibilities as low status and giving them to low-status people."
The NUT is also calling for the Government to re-examine the current multi-year teachers' pay deal, because it is not keeping pace with inflation.