Paul Ryan rules out 2016 presidential bid
US House Speaker Paul Ryan has officially ruled out making a late attempt to become the Republican presidential nominee.
"I do not want, nor will I accept the Republican nomination," he said.
Mr Ryan's name was floated as a late contender if there is a contested convention in July, as doubts persist over the strength of the candidates.
If none of Donald Trump, John Kasich and Ted Cruz can win 1,237 delegates, the convention will be contested.
The state-by-state primary contests, which come to New York next week, determine the number of delegates pledged to a particular candidate.
Mr Trump is still well ahead in the number of delegates accumulated but may fall short of the magic number required.
Only two men left - Anthony Zurcher, BBC News, Washington
In 1886 former civil war general William Sherman set the gold standard for disavowing interest in serving as US president. "I will not accept if nominated and will not serve if elected," he bluntly stated.
Speaker of the House Paul Ryan may not reach Shermanesque levels of certainty with Tuesday's statement, but the move should put the latest round of rampant speculation and rumour-mongering to rest.
The Ryan presidential boomlet was largely a result of growing desperation among Republicans who see a presidential ticket headed by the epically unpopular Donald Trump as an unmitigated disaster and by absolutist Ted Cruz as only a slightly mitigated disaster.
Mr Ryan won't be their establishment-friendly "white knight", however, and there are few others out there with the stature to pull off such an unlikely convention coup.
Former candidate Mitt Romney? Wisconsin Governor Scott Walker? At this point, anyone other than Mr Cruz or Mr Trump - the two men who have slogged through the presidential season and won the votes and delegates - appears to be pure fantasy.
At a contested convention, the delegates are free after the first ballot to back whom they want, opening the door for Texas Senator Mr Cruz or even the third candidate in the race, Mr Kasich.
Some in the party had hoped Mr Ryan would emerge as a candidate at that stage, believing he would be a more effective and less divisive figure than Mr Trump or Mr Cruz.
Speaking at the Republican National Committee headquarters in Washington, Mr Ryan - who ran as Mitt Romney's running mate in the 2012 presidential election - ruled himself out unequivocally.
But some commentators were quick to point out that he said he did not want to run for Speaker of the House last year before eventually accepting the job.