Nadine Dorries has been among the Tories’ punchiest advocates of a “hard” Brexit – arguing for Philip Hammond to be sacked, calling Dominic Grieve "treacherous", and telling colleagues lobbying for a soft departure from the European Union that they deserve to lose their seats.

But WhatsApp messages seen by BuzzFeed News reveal that Dorries needed to be convinced by colleagues about one of the eurosceptics’ firmest red lines: that Britain must withdraw from the customs union when it leaves the European Union in 2019.

As the Brexit talks reached a critical stage in October, Dorries messaged fellow Tory MPs in the European Research Group questioning one of the influential Brexit-backing group's articles of faith. "Is there any reason why, if countries outside of the EU are in the CU, we can't remain in the CU going forward?" the MP for Mid Bedfordshire wrote at 7:28am on 10 October last year.

Kemi Badenoch, one of the Tories' rising stars, replied minutes later, reminding Dorries of the ERG's position: Britain won’t be free to strike its own trade deals with countries such as the US unless it withdraws fully from the EU’s customs union.



"But, other countries who are members of the CU do," Dorries replied. "That's why I asked. Do they have exceptions? This was a question in a discussion last night. I need to find out what countries are members and have free trade agreements with other countries first."

As part of the customs union, the EU's 28 member states act as a single trade bloc, exchanging goods without internal barriers and negotiating deals with non-EU countries as one. None negotiate their own trade deals.