One unconfirmed report said he had been summoned by magistrates investigating allegations that his wife was paid for years for work she did not do.
The "fake jobs" claims have damaged his campaign and he is currently third in opinion polls.
He is due to speak publicly later.
Political rivals seized on news that he had pulled out of his visit to the Paris agricultural fair, seen as an essential appearance for any prospective candidate.
"It's like symbolically giving up on your candidacy," said Florian Philippot, close adviser to far-right National Front (FN) candidate Marine Le Pen. The FN leader is currently topping the polls for the first round of the presidential vote on 23 April.
French centre-right candidate Francois Fillon has dramatically withdrawn from a visit to a major farm fair at the last minute, raising further doubts about his campaign.