Bayrou, who was once education minister alongside the then communications minister Sarkozy between 1993 and 1995, explained his decision saying he said he had been offended by Sarkozy's lurch to the right since the first round.
This is the first time during the fifth republic that a centrist leader has moved to the left, although Bayrou stressed his decision is a personal one and not a call for his supporters to vote Hollande.
Bayrou was knocked out of the French presidential election in the first round on April 22. He polled fifth with some 9 per cent of the vote and observers were waiting to see who he would support in Sunday’s final round.
"I, personally, will vote for Francois Hollande," he said, expressing regret that his former ministerial ally had set off in pursuit of the support of the 18 per cent of the electorate that backed the far-right's Marine Le Pen.
"After a good result in the first round, Nicolas Sarkozy set off in chase of an extreme right within which we do not recognise our values and in which our deepest and most precious beliefs are battered and denied," he said.
Sarkozy has criticised Bayrou’s decision as ‘illogical’ saying it went against his own party’s policy of aiming to reduce France’s budget. He added that in 2007 Bayrou had also said he would not vote for Sarkozy which did not prevent him from going on to win the presidential election.
In 2007, Bayrou's party polled over 18 per cent of the vote coming in third in the first round of the election and he publically said he would support neither Sarkozy or the then Socialist candidate Segolene Royal in the second round.
Bayrou's belated declaration is not expected to change the electoral map.
Polls have long forecast that Hollande will win Sunday's run-off by around 54 per cent to 46.