The expansion in the electorate follows a change in the law enacted by parliament in 2022, which removes a previous restriction on UK nationals voting if they have resided overseas for more than 15 years.
It is the most major alteration to the voter registers since a 1928 law provided women equal voting rights, as well as a 1969 move to decrease the voting age from 21 to 18.
Britons around the world will now be allowed to register to vote online, regardless of how long they have been abroad.
Once registered, individuals are also permitted, under UK election law, to donate to political parties and campaigners.
Around 233,000 overseas voters registered for the most recent election in December 2019, representing a large increase above previous contests due to Brexit.
The government predicts that change will enfranchise approximately 3.5 million people, roughly double the 1.3 million votes won in the 2016 referendum on European Union membership.
It is also larger than the disparity in vote totals between Britain's two main parties, the Conservatives and Labour, in five of the last six general elections.
However, in UK elections, parliamentarians are elected in 650 individual constituencies using the first-past-the-post system, regardless of the parties' total vote count.
It is uncertain how many of the newly eligible 3.5 million UK nationals living abroad will actually register to vote.
They will be required to supply information on the address and time they were last registered to vote or lived in Britain.
Local authorities, which are responsible for the electoral roll in their areas, must be able to verify an applicant's identification and past connection to the area, according to the Electoral Commission.
Unlike in a few other countries, there is no opportunity for in-person voting overseas, and all ballots in the United Kingdom must be cast by post or proxy.
The Electoral Commission is initiating a marketing campaign and collaborating with partner organizations to promote awareness about the rule change.