Election 2017: Places with the lowest voter registration
More than a third of people in some UK constituencies are not registered to vote, according to official figures.
Data showed in some areas there were tens of thousands of adult residents not on the electoral roll.
The Electoral Commission has warned that about seven million people across Britain who are eligible to vote are not registered.
The deadline to register in time for the UK general election is 23:59 BST on Monday.
The BBC England Data Unit combined the latest Office for National Statistics data for December 2016, showing the number of registered voters per constituency, with the official population estimates.
It showed that just over half the people (58%) of voting age estimated to live in with the Cities of London and Westminster constituency are registered - about 61,000 out of an estimated 106,000 adult residents.
The area includes the financial heart of London and the city of Westminster, represented by one MP.
And in Kensington there were 37,000 more adult residents than registered voters.
Both constituencies contain very affluent areas of London with high-earning constituents. They are also areas with very high levels of non-British residents, who would not be eligible to vote.
This compares with a difference of fewer than 100 between registered voters and adult population in Congleton in Cheshire.
The difference in some areas can be explained by immigration as people who are not British, Irish or qualifying Commonwealth citizens are not eligible to vote.
In Westminster, around 31% of people are not UK citizens and in Kensington and Chelsea it is as high as 37%. For other areas it is more difficult to say how much of the difference is down to immigration because of the way data on nationality is recorded.
Rob McNeil, of the Migration Observatory at the University of Oxford, said: "It is difficult to get a precise picture of migrant population of a constituency as the data tends to be broken down by local authority area, many of which are divided up when it comes to seats in Parliament."
Congleton, which has the highest proportion of registered voters, is part of the Cheshire East council area, where just 4% of residents are not UK nationals. It is considered by the council to be "generally a prosperous town" and has a strong manufacturing tradition, with major employers such as Siemens, but does have some areas of high deprivation.
The latest official estimates for population by constituency are based on mid-2015.
BBC News therefore included in its analysis people who were 17 in 2015, as they would be of voting age by the time the electoral registration figures were compiled in December 2016.
They do not take account of anyone who may have died since the middle of 2015. And the figures do not exclude about 90,000 prisoners across the country who are not eligible to vote.
More than two million people have applied to register to vote in the month since Theresa May announced plans for a snap general election on 8 June.
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He said: "We are campaigning to show people that if they voted, our world would change.
"We are making a big push and are hoping for tens of thousands of people to sign up today. It only takes about three minutes."
Emma Hartley, head of campaigns at the Electoral Commission, said: "It's important that anyone who is eligible to vote has the opportunity to participate in this election. However, if you're not registered to vote before midnight on 22 May, then you won't be able to have your say. We're urging anyone not yet registered to do so straight away. Registering to vote is easy to do online and only takes a few minutes."