Postal voting registration - which had been set to close on Wednesday - has been extended by five days to next Monday.
The electoral commission announced the extension this afternoon, saying about 4400 had registered so far.
Postal voting gives people who cannot vote in person - for example those with health or disability barriers - the option to do so remotely.
The commission said completed voting papers should be returned by post by Wednesday 14 October, or dropped off at a voting place by 7pm on election day.
They said arrangements could still be made after 28 September for people unable to vote in person, by calling 0800 36 76 56.
Read more about the 2020 election: