Boris Johnson is on a collision course with Brussels when the second phase of Brexit negotiations open — as a former Downing Street aide warned that Whitehall is not “match fit” for the talks.

The prime minister will reject calls from EU leaders for Britain to accept a “level playing field” on regulations, which would mean the UK adopting many of the same rules as the rest of the bloc even after Brexit.

A senior figure in government dismissed claims that Johnson would use his new majority to abandon Brexiteers in the European Research Group (ERG) and pursue a softer Brexit. The source said it was “crazy” for people to say Johnson would allow an extension to the transition period, which is due to expire