Sarkozy only said: "I will no longer be minister come the election."
His bid to become the first immigrant's son to lead modern France faces competition from socialist Segolene Royal, who is seeking to become the first woman to run the euro zone's second-biggest economy.
Polls show them neck-and-neck, but a recent surge by veteran far-right leader Jean-Marie Le Pen means neither is assured of a place in the May 6 run-off ballot.
Le Pen stunned France in the 2002 presidential election by beating Lionel Jospin, the former socialist prime minister, into third place and out of the race.
Commentators say it would take a similar kind of political change to prevent the UMP's 300,000 members picking Sarkozy.
The centre-right's primary is small scale compared with the six weeks of rallies and televised debates the opposition socialists organised before picking Royal this month.
The contest has been seen as a sop to internal critics and a response to polls showing UMP members wanted a socialist-style primary, something Sarkozy rejected given the mainstream right's record of debate, internal warfare and electoral debacle.
Michele Alliot-Marie, the defence minister, has criticised Sarkozy's sometimes strident style and policies such as affirmative action for minorities. She has until the end of the year end to announce whether she will run against him.
An Ipsos poll of UMP supporters in the Le Point news weekly showed 78 per cent support for Sarkozy, 13 per cent for Alliot-Marie and nine per cent for Dominique de Villepin, the prime minister.