About 60,000 people in Brighton and Hove are at risk of losing their vote as electoral registration reforms take effect.
Brighton and Hove City Council is writing to residents to explain the changes.
The council said: “The way people register to vote is changing across the UK.
“In Brighton and Hove around 60,000 residents may need to take action to join the new register, even if they were registered in the past.
“The registration system changed in June and this is the biggest reform to the voter registration system in a century.
“The new system is called individual electoral registration.
“Previously, the ‘head of household’ was responsible for registering everyone who lived at an address but now every individual is responsible for their own voter registration.
“The new system means that for the first time people are now able to register online at www.gov.uk/register-to-vote.
“To register, you will need to provide a few more details than previously required, including your national insurance number and date of birth.
“This helps make the electoral register more secure.
“Most people who are already registered have been transferred automatically to the new register.
“However, around 60,000 will need to re-register in the Brighton and Hove area.
“There are also people in the area who are not registered to vote at all and so need to register to have their say in elections.”
Those people can also register under the new system at www.gov.uk/register-to-vote.
The council said that people should look out for a letter between Monday 14 July and Wednesday 23 July which should say whether action is needed to re-register.
People without access to the internet can use their local library or the council’s customer service centres in Bartholomew Square and Hove Town Hall. Staff have been trained to help people to register to vote.
For more information about the changes, visit www.brighton-hove.gov.uk/vote or www.gov.uk/yourvotematters.
People can also find out more by contacting the council’s Electoral Services Team on 01273 291999 or by emailing electors@brighton-hove.gov.uk.