Betting market odds on the U.S. presidential election have begun to tighten once again after clearly flipping overnight in favour of Republican President Donald Trump over Democratic candidate Joe Biden, according to data from three aggregators.

British-based Smarkets exchange is giving Trump and Biden an almost an equal chance of winning as of 1000 GMT, with Trump sitting at a 51% chance. The odds on Trump had risen to as much as 80% overnight.

Odds for a Biden win have fallen to 49% from 61% on Tuesday.

The shift came after Biden overtook Trump in the battleground state of Wisconsin with an estimated 89% of the vote tallied so far. Trump has 49% and Biden has 49.3% of vote, according to Edison Research.

Trump earlier falsely claimed victory over Biden on Wednesday with millions of votes still uncounted in a White House race that will not be decided until a handful of states complete vote-counting over the next hours or days.

The flip in betting markets overnight was mainly "driven by Trump seemingly holding the key swing state of Florida as a result of a very strong showing for him in the Miami-Dade county with a large Cuban population," said Patrick Flynn, political analyst at Smarkets.

Bettors on British betting exchange Betfair were giving Trump a 62% chance of winning a second term in the White House at 0800 GMT, up from 39% when polls opened on Tuesday morning. Biden's odds of a win on Betfair had fallen to 38% from 61% earlier.

"With thousands of votes still to be counted, it remains a coin flip over who will be victorious, but for now Donald Trump is the punters’ pick," Betfair spokesperson Sam Rosbottom said.