Hillary Clinton has opened up a slim lead across several decisive swing states among early voters in the presidential election – despite the announcement of a fresh FBI investigation into the former Secretary of State.

At least 21 million have cast their votes in the US, including in the states likely to determine the result.

The turnout in the states that were previously on a knife-edge will comfort the Democratic campaign after this year's so-called 'October surprise', a term used to refer to significant, late-breaking stories that could swing an election.

Almost a quarter of potential voters have cast their ballots in Nevada, Colorado and Florida. Although their votes will not be counted until the day of the election, registered Democrats are outstripping Republicans in some key areas.

In Colorado, Democrats are exceeding their performance in the 2012 election. Although Barack Obama ultimately claimed the state, at this moment in the last election, the Democrats were behind Republicans by about 19,000 votes.

This time, 39 per cent of the ballots cast have been from registered Democrats and only 35 per cent from Republicans.

Similarly in Nevada, Democrats have so far outpaced Republicans by seven percentage points.

Ms Clinton is also ahead in North Carolina, a crucial state that has oscillated between red and blue. Democrats are far ahead in the number of votes cast so far, with 43 per cent compared to the Republicans 31 per cent.

And, in a further surprise twist, Democrats have also improved their position in conservative Utah, where polling had shown the race was neck-and-neck.

In an election already riddled with scandal, interviews with early voters suggested news of the fresh FBI investigation had failed to persuade people to switch from Ms Clinton to Donald Trump.

Fernando Gonzales, a 25-year-old who voted Democrat in Miami, told the The New York Times: “The October surprise doesn’t mean anything. The bar’s been lowered so much, you can’t lower it any more.”

Mr Trump's best gains were in Iowa. So far, 38,280 more Democrats than Republicans have voted, but that is a narrower advantage than at this point in 2012, when Democrats outpaced Repulicans by 53,719.