LONDON (Reuters) - A total of 433,695 Britons applied to vote in the country's European Union referendum during a 48-hour extension period granted earlier this week after huge demand close to the original deadline overwhelmed the registration website.

The late surge in registration applications, announced by Prime Minister David Cameron's spokeswoman on Friday after the extended deadline had passed, brought the total number of registrations in the last week to 1.5 million.

Not all of those will be new voters, with some using the system to check they are correctly registered.

Turnout is expected to be important in determining the outcome of the close-fought referendum, with young people considered to be more pro-EU but also less likely to be properly registered vote and to actually cast their ballot.

(Reporting by William James; editing by Stephen Addison)