The claim: The six strongest pro-Leave constituencies and the six strongest pro-Remain constituencies all have Labour MPs.
Reality Check verdict: The best estimates available suggest that two of the six most pro-Remain constituencies have SNP MPs, while three of the six most pro-Leave constituencies have Conservative MPs.
Shadow home secretary Diane Abbott said on Friday that any leader would be challenged by the situation in which the six most passionate pro-Leave and pro-Remain constituencies all had MPs from their party.
The EU referendum vote nationwide was close, with 48.1% voting Remain and 51.9% Leave.
But Remain voters tended to be clustered in big cities and in Scotland and Northern Ireland, while the Leave vote was more evenly spread.
This means that while a small majority of the country voted Leave, a large majority of the country's 650 parliamentary constituencies did so.
The votes in June were counted by council, of which there are far fewer - about 400.
This is because cities tend to have one council but several Westminster constituencies.
This means in most cases we cannot say exactly how each parliamentary constituency voted.
However, Chris Hanretty, an academic at the University of East Anglia, has used the results for local authorities to make detailed estimates for each constituency.
These are based on other things we know about the area, such as age, education, ethnicity and income, and how strongly those factors correlate with a vote for Brexit in the places where we know the vote at constituency level for sure.
Mr Hanretty estimates that most constituencies where Remain support was the highest did indeed vote Labour in 2015, although Glasgow North and Edinburgh North, both represented by the SNP, are exceptions.
But that's not the picture in the areas that most passionately backed Leave.
The Brexit vote was strongest in parts of eastern England, especially Lincolnshire, which backed the Conservatives in 2015.
Mr Hanretty estimates that Conservative-held Boston and Skegness was the constituency with the highest Leave vote.
It covers the Boston council area, which recorded the strongest vote to leave the EU of any local authority, and part of East Lindsey, another overwhelmingly pro-Brexit area.
It's true that some places where Leave support was strongest voted Labour in 2015, such as Doncaster North, Ed Miliband's constituency. But the Leave vote was highest in Conservative parts of eastern England.