"We now face a series of choices about the kind of Brexit we want,” said George Osborne last week. “And when it comes to leaving the EU customs union versus what we might gain from doing a trade deal with America … the sums don’t stack up.”

The “sums” the former chancellor cites were contained in an early draft of an internal civil service document, “EU Exit Analysis – Cross Whitehall Briefing”, that was leaked last week.

Written by Treasury and Cabinet Office officials, the report painted leaving the EU in the worst possible light. The leak was also timed to inflict maximum damage – as the Lords were debating the EU Withdrawal Bill and the PM was 5,000 miles away in Beijing, ahead of this week’s crucial Cabinet meeting on Brexit.

Three post-Brexit scenarios were examined: a close “Norway” deal with the EU, a “Canada-style” negotiated trade deal and the “WTO option” with no EU trade deal...