Washington: Donald Trump has for the first time been named as the favourite to win the Republican nomination by a betting agency.

Though Mr Trump has long led national polls in the United States, betting markets, considered by some a more reliable predictor this far out from voting, have consistently predicted that establishment candidates would overtake him.

"The market for Republican nominee has been the tightest betting contest in the 2016 US presidential election so far and this the first time Trump has been favourite, which could well present a significant shift at a pivotal time in the campaign," analysis on the Betfair website says.

Betfair calculates that the odds of a Trump victory are 15/8 or 34 per cent, placing him in the lead for the first time, ahead of Marco Rubio​ who had led since October and has now widened to odds of 11/5 or 31 per cent.