British-based US voters could swing key states in next month's race for the White House, an Oxford University study has found.

With polls showing the election still close in some of the battleground states despite a series of scandals engulfing Republican candidate Donald Trump's campaign, the report says the estimated 2.6 million US voters living overseas could have a major effect on the result in some states.

Canada is host to the biggest number of expat Americans with 661,000, Britain is home to the next largest cohort, 306,000, then comes France with 157,000, followed by Israel, Japan, and Australia, which all have more than 100,000 voter-age American residents each.

While Mr Trump still appears to be in contention in some of the Northern “rust-belt” swing states, Hillary Clinton is now pouring resources into traditionally Republican-leaning areas such as Arizona.

The study finds that Arizona, along with North Carolina, Georgia, and Iowa could go Democrat with the backing of enough foreign-based voters on November 8.

Report co-author Dr Patrick Andelic, research associate at the Rothermere American Institute (RAI) at Oxford University, said: “Canada, Britain, France and Israel all play a substantial role in US elections now.

“While current polling places Clinton in a commanding lead, the volatile nature of the race so far means that anything can still happen - and if Britain's recent general election and Brexit result have shown us anything, it's that one shouldn't call a winner until the votes have all been counted.

“Overseas voters proved crucial to George W Bush's victory in 2000 and they may make a critical difference in 2016. Political parties ignore this hidden constituency at their peril.”

RAI director Dr Halbert Jones, co author of the report, said: “Our analysis shows that, based on recent polls, Trump might need a majority of just 5,600 among Ohio's overseas voters to win the state, and a majority of just 7,100 among overseas voters to win Nevada.

“But if the national vote swings further behind Clinton, the overseas vote could mean she takes the presidency with a rout rather than a slim victory - helping the Democrats to snatch prizes like Georgia, Iowa and Arizona.”

The significance of Mexico is disputed with a US Government study suggesting there are just 65,000 US voters living there, while the 2010 Mexican census puts the figure closer to 200,000.

However, the report suggests with such a huge number of US-born children living in Mexico - more than 800,000 - the country is set to play an increasingly important role in American elections in the future.

The report notes: “By far the most dramatic instance of overseas voters influencing a presidential election came in 2000, when delayed overseas ballots gave George W Bush a narrow 537-vote lead after the Florida recount was stopped by the Supreme Court.

“Had that election been decided on ballots that had arrived by the original November 26 deadline, Al Gore would have won the state, and the presidency, by 202 votes.

“Similarly, overseas voters may have played a decisive role in some very close Senate races in recent years.”

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With only 5% of eligible overseas voters casting a ballot in the 2012 White House race, the report states the impact this year could be far greater, especially after a major “Stop Trump” registration drive was launched.