JUST over 30,000 voters in 50 key constituencies will decide our next PM — and the fate of Brexit.

If Boris Johnson is to triumph he needs 25 marginal Labour and Lib Dem seats to swing his way.

But he also has to cling on to a further 25 constituencies the Tories hold with slim majorities.

The Sun can reveal Boris Johnson’s lucky number is . . . 32,195.

That is the number of voters he needs to back him in the 50 key constituencies that will decide the general election.

On December 12 voters across the UK go to the polls to elect 650 MPs to the House of Commons.

But experts say constituencies are likely to change hands in only a fraction — meaning voters in those seats will chose both the next Government and the fate of Brexit.

The Sun’s analysis reveals there are 25 target seats that the Tories are most likely to take off opposition parties – 20 from Labour and five from the Lib Dems.

We looked at the size of the majorities, the efforts the three main parties are making in them and the size of support for Brexit – which in all 25 is substantial.

There are a further 25 seats, currently held by Conservative MPs by slim margins, that Mr Johnson needs to cling onto.

In the Labour-held seats targeted by Mr Johnson, just 17,071 voters have to swing to the Tories to turn them blue. In the Lib Dem constituencies it requires a swing among 4,349.

In the Conservative-defended seats, only 10,775 voters must stick with the Tories to keep them blue.

That makes 32,195 votes in total. If they all back Mr Johnson he will be back as PM with a majority of 32 — big enough to ensure his Brexit deal is passed by Parliament and we leave the EU by the January 31 deadline.

The ultra-tight election campaign entered its final three weeks today as:

• Ex Labour PM Tony Blair revealed he does not want his party to win the election — and called for moderates to vote tactically.
• The cost of Labour’s plans to hold two further referendums was revealed to be at least £150 million, with the Tories highlighting Jeremy Corbyn’s indecisiveness on Brexit.
• Brussels was warned of the risk of long-term damage to ties with Britain if it takes a “hardball” approach to post-Brexit trade talks with Mr Johnson.
• Theresa May would get more than £20,000 under Mr Corbyn’s plans to give £58billion to women hit by state pension age changes.
• Corbyn was accused of “rubbishing our history” as he announced plans to force children to be taught the British Empire was “evil.”
• Channel 4 plans to humiliate Mr Johnson for snubbing its election debate on Thursday — by representing him with an empty chair.

All parties will today urge potential supporters to ensure they are registered to vote. The deadline for registration is 11:59pm tonight.

If Mr Johnson fails to win a Commons majority for the Tories Mr Corbyn will almost certainly be catapulted into power at the head of a “rainbow coalition.”

During the campaign commentators have stressed Mr Johnson’s need to break down the “red wall” of Labour seats that stretch across northern England.

But while many Tory targets are in that area, the battleground seats actually stretch across the country — in Scotland, the North West, the West Midlands, Wales, Norfolk, Essex and Kent.

Polling expert and Strathclyde University Professor Sir John Curtice said: “The outcome rests on the very small range of seats that the Tories must hold — as well as the ones that the Tories must gain.

“It is particularly close because if the Tories fall short by only a very few MPs, they cannot form a coalition with anyone so will not be able to govern. It is a knife-edge election — and very unusually so.

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“Very small movement around the 326 mark produces a very difficult outcome politically — the difference between night and day.”

Labour and Leftwing campaign groups have carried out a massive operation to try to persuade anti-Tory and pro-Remain voters to register over the last weeks, targeting students especially.

A record 308,000 signed up on National Voter Registration Day last Friday. To vote by post in England, Scotland or Wales, you must register before 5pm.

• Register to vote by going to www.gov.uk/register-to-vote

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