Giuliani, who was until recently a favorite to head the Republicans in U.S. presidential elections due next fall, made the announcement at the Ronald Reagan Library in Simi Valley, California and said he was endorsing John McCain for the Republican nomination.
Giuliani told journalists: "Today I am officially announcing my withdrawal as candidate for the President of the United States," adding "I'll campaign with John as much as I can, in any place he thinks I can be helpful. I'm fully aboard, 100 per cent. This will become for me as important as my own campaign was."
According to polls, up until the middle of last fall Giuliani was considered to be a favorite among Republicans and was backed by many well-known politicians.
But analysts believe the former mayor's campaign strategy of concentrating all his strength on winning Florida was a mistake.
And it was Senator John McCain who won the Republican primary in Florida Tuesday with 36% of the votes with his rival Mitt Romney polling 31% ahead of crucial primaries being held in over 20 states on February 5.
The 71 year-old Arizona senator is now firm favorite to run as the Republican candidate in the presidential elections.
