Voting has already begun in the US presidential election - but it could potentially be weeks before we know who has won.

The polling stations will open on Tuesday, but millions have already sent in postal ballots in one of the tightest races in living memory.

Republican candidate Donald Trump is seeking to return to the White House four years after losing to Joe Biden.

For the Democrats, Kamala Harris is looking to create history by becoming the first ever female president.

Here is what you need to know.

How does the American electoral system work?

Each of the country’s 50 states is allocated electoral college votes depending on their size.

The winner of a state has its electoral college votes added to their overall tally.

With a total of 538 electoral college votes up for grabs, the target for Trump and Harris is to reach 270, giving them an unassailable lead over their rival.

In 2016, Trump beat Hillary Clinton by 306 to 232, while in 2020 Biden beat Trump by 306 to 232.

What are the key states to watch?

The result will ultimately be decided by the results in seven key swing states - Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin.

Between them, they have a total of 93 electoral college votes.

Four years ago, all of the swing states bar North Carolina were won by Biden, handing him the presidency at the age of 78.

When will we start to see the results?

The first polls will start to close at 11pm UK time on Tuesday night, with the last closing at 6am on Wednesday.

Many of the states will be won comfortably, meaning the results there are expected to be announced relatively quickly.

However, these results will be able to give us an early indication of voting patterns across the country.

Voting in the swing states will close between midnight and 3am, after which the major US news network will start preparing to call the overall result.

But due to the tightness of the race, it may be some time before the nationwide picture begins to become clear.

When will we know who has won?

Given the closeness of the opinion polls, and the likelihood of legal challenges, no one is expecting the winner to be officially announced any time soon.

Four years ago, it took until the Saturday morning for Joe Biden’s victory to be confirmed - although it was still not accepted by Donald Trump, who continues to claim that he was cheated.

Once every vote has been counted and the result has been verified, a new US Congress will meet on January 6 next year to count the electoral votes sent in from each state.

When does the new president start work?

Either Harris or Trump will be inaugurated on January 20 in the grounds of the US Capitol complex.

He or she will swear to uphold the US constitution before giving their inaugural speech.