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?