How many Labour Leave voters are there?

Nigel Farage, leader of the newly-formed Brexit Party, declared on Twitter, referring to Labour’s deputy leader Tom Watson MP:

The second referendum that [Watson] backs is a total insult to 5 million Brexit Labour voters.

The Twitter account for ‘Labour Leave’ made similar claims.

Are there really 5m Labour voters who backed Leave in the 2016 EU referendum? Survey projections suggest there are less than 4m such voters.

How did Farage get to 5m Labour Leave voters?

It is unclear what source the Brexit Party leader has used for this claim.

Following the 2016 EU referendum, there were multiple surveys which looked at each party’s voters in 2015, and what share backed Remain or Leave.

Excluding people who did not turn out to vote in 2016, YouGov’s post-referendum survey suggested 35% of 2015 Labour voters backed Leave. For the Lord Ashcroft-badged survey (which was conducted by an actual polling company), that figure was 37%.

One possible method is the 37% figure was multiplied by Labour’s 2017 vote count (about 12.9m), arriving at 4.8m, before rounding up to 5m.