Theresa May's plan for a customs partnership with the EU is "unviable", HM Revenue & Customs believes, as it emerged Mrs May is prepared to accept the Brexiteers’ favoured “Max Fac” option.

HMRC believes the customs partnership idea is "incredibly complicated" and impractical, Whitehall sources have told The Telegraph, and the idea it could be used to solve the Irish border problem is "for the birds".

A customs partnership with Brussels - which involves collecting tariffs on behalf of the EU - is opposed by a 6-5 majority of Mrs May's Brexit “war Cabinet”.

Leave campaigners including Boris Johnson and Michael Gove prefer the alternative "Max Fac" option - short for maximum facilitation - which relies on new technology and trusted trader schemes to avoid a hard border with the EU.

In a clear softening of her opposition to Max Fac, Mrs May has made it clear to senior Cabinet ministers that if it is the best way to reach an agreement “then so be it”.