Polling booths have opened in the UK for US expats wishing to vote for a Democratic presidential candidate.
Democrats Abroad is treated as a state and will send a similar quantity of delegates to the Democrat National Convention as Wyoming and Alaska.
Republican voters can only participate via individual absentee ballots.
Twelve US states cast votes for candidates from either the Republican or Democratic parties on Super Tuesday.
The contest is seen as a make or break moment for presidential hopefuls.
The so-called Global Presidential Primary – during which US expats can vote for their Democratic hopeful – opened to electronic and postal voting in January, but Tuesday sees the start of voting at polling stations.
The five UK voting centres – along with another 116 worldwide – will be open until 8 March.
The Democratic candidate needs to obtain 2,383 delegates to win the nomination – and 21 will come from Democrats Abroad.
Larry Sanders, the brother of Democrat candidate Bernie Sanders, will cast his vote at the Rothermere American Institute at the University of Oxford.
Prof Jay Sexton, director of the institute, said Democrats Abroad was “very well organised and decently funded” and could play a significant part in choosing the party’s candidate.
“I know Republicans Overseas are seeking to organise something similar in the future” he added.
Prof Sexton continued: “[And] it’s not just the primaries where these American expats should be taken seriously. Overseas voting was critical to putting George W Bush into the White House in 2000, and if things are tight, could be just as important in the 2016 election too.”
Some expat voters in the UK also used social media to encourage participation.
There are an estimated 8.6m US citizens living abroad, and according to the 2011 census, more than 170,000 are in England.
Last week, the Democratic National Committee chairman, Governor Howard Dean, visited London to raise funds and mobilise the expat vote.