Tony Blair will miss the vote to decide which city stages the Olympic Games in 2012, denting London's chances of success.
Leaders of the London bid have conceded that the Prime Minister cannot attend the International Olympic Committee (IOC) meeting in Singapore on 6 July because it coincides with the start of the G8 summit he is hosting in Gleneagles, Scotland.
The clash is a setback for London's chances because of the positive impact Mr Blair made within IOC circles as the only head of state from a bidding city to attend the Athens Olympics.
The Prime Minister, who as the star of the bid launch at the Royal Opera House in Covent Garden last January urged the IOC to "give us a chance", was considered the perfect figure to deliver the final message just before the secret vote.
Bid leaders have not ruled out the possibility of Mr Blair attending the eight-day IOC session earlier. But one voting member warned his absence when each of the five cities makes an hour-long presentation could be critical. Information gathered by bidding cities of the intentions of 117 IOC members eligible to vote suggests Paris holds the lead by just five votes.
The IOC member said: "If it is a close vote - and there is every indication that London and Paris will go head to head - the presence of a head of state in front of the IOC panel could be crucial. He or she stands up and says, 'My government supports the bid and we will be writing the cheques'. You can bet your bottom dollar that [French president Jacques] Chirac will be there."
Leaders of the Paris bid are thought to be going to extraordinary lengths to ensure that M. Chirac - a veteran of three bids - will be at the vote. Paris will make the first presentation and M. Chirac could say his piece and take advantage of the time difference to fly to the G8 summit. That is not an option for Mr Blair as London will make the fourth presentation and would not get back in time.
Jose Luis Rodriguez Zapatero, Spain's Prime Minister, is expected to represent Madrid, as are King Juan Carlos and Queen Sophia. The Russian Prime Minister, Vladimir Putin, has said he will attend, hoping to boost Moscow's flagging chances. It is unlikely that President George Bush will be present, although he may not be missed because of the attendance of New York's mayor Michael Bloomberg, whose charismatic presence at a recent meeting of the European Olympic Commission has boosted his city's chances.