John Illingworth, a primary head from Nottingham, was one of three candidates competing to succeed Doug McAvoy as general secretary of the National Union of Teachers.
The former NUT president had the backing of two of the union's left-wing factions, the Campaign for a Democratic and Fighting Union and the Socialist Teachers' Alliance. They have already begun to search for a new joint candidate.
Mr Illingworth said: "I have been unwell for some time and I have had medical advice that I have got to ease off. I cannot do the job of head teacher and campaign at the same time so I have decided that it is both in my own interests and those of the union that I stand down now."
He said he was disappointed because he stood a good chance of winning. He leaves Steve Sinnott, deputy general secretary and John Bangs, the NUT's head of education, still in the race.
Mr Illingworth had managed to build a good profile in the union during his year as president and through his campaigning against national testing.
Members of the CDFU and STA have begun informal discussions to find a successor, with several names already in the frame. They are expected to come to a conclusion at a meeting to be held early next month. An early front-runner is Christine Blower, another former president from the CDFU, who put up a respectable performance when she stood against Mr McAvoy in the last NUT leadership election in 1999.
Some leading figures on the left believe that selecting a woman as a candidate could give them an edge in the contest.
The other female possibility is Carole Regan, a former president married to executive member Bernard Regan whose name has also been mentioned.
Other contenders include Ian Murch, who ran against Mr McAvoy in 1989, Alex Kenny, STA convenor, and two local London secretaries, Kevin Courtney and Martin Powell-Davis.
Whoever does enter the race faces a gruelling challenge to catch up. Their rivals have been building support have their own websites since announcing their candidatures at Easter.
But formal nomination papers for the election will not be sent out to local branches until January, to be returned by May 3. The ballot itself runs from June 7-28.