Newly-registered voters and those abandoned by their previous party hold the power to swing 71 constituencies in England and Wales, the Bureau Local can reveal.
Our analysis has uncovered new battlegrounds in seats with large numbers of voters without a known party affiliation – new voters and 2015 voters whose party is not standing a candidate this time around.
Nearly 50,000 (49,984) newly-registered voters and 400,000 (395,672) former UKIP and Green supporters have been identified as key to constituencies where the margin is tight.
The seats are spread across England and Wales and include the fiercely contested battlegrounds of Derby North and Croydon Central, while also identifying others previously thought safe such as Colchester.
New voter power
The analysis has found 50,000 new voters in 19 seats where the increase in voter registration is greater than the majority won by the victorious candidate in the last election.
Less than 7,000 of these newly-registered voters have the power to swing six key Conservative seats: Derby North, Thurrock, Croydon Central, Twickenham, Plymouth Sutton & Devonport, and Crewe & Nantwich.
If Theresa May loses six seats without winning any new ones, she will no longer have a parliamentary majority.
Three further Tory-controlled seats - Colchester, Telford and Bedford - also have more new voters than the margin.
However these are seats where UKIP had a candidate running in 2015 but does not this time - and the number of people who voted UKIP last time outweighs the newly-registered voters.
Tory MP Oliver Colvile who holds Plymouth Sutton & Devonport has the most at stake. Colvile, who has held the seat since 2010, has a majority of just 528 - but more than 3,000 new voters have registered in this constituency for the upcoming election.
Crewe and Nantwich in Cheshire has been held by the Conservative minister Edward Timpson since 2008. Yet with the new registration figure at more than 1,000 more than the majority, this seat has potential to turn red if enough of these new voters back the Labour candidate Laura Smith.
Additionally, Sir Vince Cable has a chance of returning Twickenham to the Liberal Democrats.
In 2015 Cable lost the seat, which he previously held since 1997, to Conservative Tania Mathias by just 2,017 votes. The constituency now has more than 3,000 newly-registered voters.
Both the Green Party and UKIP no longer have candidates standing which adds an additional 3,000 UKIP and nearly 2,500 Green voters to fight for.
Over a million UKIP votes up for grabs
In addition to new voters, there are more than 1.2 million UKIP votes to be won in the 207 constituencies where the party has withdrawn candidates.
Over 370,000 of them could play a deciding role, finding themselves in 61 constituencies where the UKIP vote was greater than the margin of votes between the first and second candidate in 2015.
This is a potentially crucial group of seats with the Tories and Labour both holding 29 of them and the Lib Dems three.
While general opinion is that the Conservatives will gain most of UKIP’s losses, the past voting history in these seats does not offer a convincing argument to back up the claim.
Among those seats held by the Tories, all but one has been won by another party at least once since 2005.
Among the casualties the Tories could suffer should Labour cut deep enough in the UKIP electorate is Brighton, Kemptown - the seat of City Minister Simon Kirby.
The minister won the seat by a narrow 690 vote majority over Labour in 2015 - making it a key battleground in this election that UKIP and the Greens have both pulled out of in an effort to boost the main candidates from the right and left.
Should Labour gain the vast majority of the 3,190 Green votes left undefended from 2015 and convince some the 4,400 UKIP electorate, this could end up as a tight victory for Labour.
There are several other seats, including Crawley, Stevenage and Norwich North, that could feasibly turn red, as they were in previous elections, should the Labour candidate in those constituencies lure back the abandoned UKIP electorate.
Thousands of Greens votes left for Labour to recycle
In addition to the former UKIP votes, 138,000 Green votes have also been left for others to pick up in 94 constituencies.
In 13 of those seats, the 2015 Green vote share is greater than the majority held by the incumbent. This gives 24,000 voters the power to swing seats.
Previous Green voters could prove to be the most valuable to Labour in Derby North, Gower and Halifax.
Those three constituencies were all decided by less than 500 votes in 2015 – placing them among the most marginal constituencies in England and Wales.
With small margins, the more than 1,000 votes the Greens collected in each of them could be crucial for Labour.
Derby North in particular would benefit from former Green votes. It is one of the seats currently held by the Tories by a margin of 41 votes, and now has 1,600 former Green voters without a candidate. It also has 125 newly-registered voters.
The model in this analysis used data on England and Wales from the Office of National Statistics, the Cabinet Office and the British Election Study in order to identify where newly registered voters can tip the margin in individual constituencies.
Actual registration figures for over 50 constituencies were obtained by the Bureau to report exact numbers.
The analysis identified seats where the increase in voter registration is greater than the majority in the last election and seats where UKIP and Green candidates are no longer standing but their past vote share is greater than the 2015 margin.
Modelling developed by Zsolt Kiss
Additional reporting by Callum Adams, Matty Edwards and Ed Fairhead