LONDON (Reuters) – The number of Britons registering to vote before a midnight deadline expired on Tuesday was up 31 percent compared to a similar period before a 2017 election, with data showing almost 4 million registrations ahead of the Dec. 12 election.

The outcome of the election, which will determine how, when and even whether Britain leaves the European Union, remains highly uncertain. Prime Minister Boris Johnson’s Conservatives are ahead in opinion polls but three published so far this week have shown their lead narrowing.

Government data showed 3.85 million registrations to vote since parliament agreed on Oct. 29 to hold an election.

The Electoral Reform Society (ERS) said this represented a 31 percent increase on the 2.9 million applications received in the comparable window before the snap election in 2017.

The ERS, a coalition of civil society groups and campaigners, said 67% of registrations were from those aged under 35, slightly lower than the 69% seen in 2017.

It is unclear how many new voters will be added to the electoral roll because many of those who applied will already be registered to vote. The ERS said that based on 2017 data, it estimated 1.4 million applications would turn out to be duplicates.

Reporting by William James and Kylie MacLellan; editing by Stephen Addison