Audited figures show the ATL added 15,117 members last year - 9 per cent up on the year before. The previous year saw a 15,000 rise, officially the biggest increase in membership of any union in the United Kingdom.
All classroom associations reported increased recruitment. The National Union of Teachers consolidated its position as the largest union, adding 7,000 members and extending its lead over the National Association of Schoolmasters Union of Women Teachers, which added only 1,238.
The Secondary Heads' Association is also growing, while the National Association of Head Teachers saw a fall - though the small drop of 338 suggests the decline of more than 3,000 members in 1998 has been arrested.
The figures, submitted to the trade unions' certification officer,Graham Osborne, are for December 1999.
ATL general secretary Peter Smith - still the highest-paid of the union bosses - said growing insecurity was the reason more teachers were turning to unions.
"Teachers feel immensely vulnerable," he said. They were turning to the ATL because they were "bored with soundbites"... "On major policy issues such as teachers' pay, they want more than kneejerk reactions. They want a union that will give them real, solid help in their professional lives."
PAT has the greatest proportion of female members at 86 per cent. NASUWT attracts the most men - two members in every five.
Women constitute 52 per cent of headteacher union membership, compared with 70 per cent classroom membership, underlining the glass ceiling effect - particularly in secondaries. Men make up two-thirds of the Secondary Heads' Association.