May's Brexit deal with the EU will be almost impossible to get out of, the Attorney General has warned amid mounting Cabinet opposition.
Geoffrey Cox is said to have made clear that ministers will not be able to change the customs backstop once the UK is signed up to it.
Eurosceptics are demanding an "exit mechanism" that enables the UK to break off the backstop, which will come into force in March 2021 if the UK has still not signed a deal with Brussels.
It is designed to avoid a hard border with Ireland, but Eurosceptics fear that it will leave the UK in a state of "permanent vassalage" with the EU.
