Theresa May’s decision to challenge Jacob Rees-Mogg over the future of Northern Ireland comes amid mounting nerves in No 10 about the future of the union after Brexit.

Downing Street and key ministers have been shown polling from October that suggests opinion in the province is drifting towards a united Ireland. Another finding suggests that leaving the EU with no deal on the border could shift voters in Northern Ireland decisively in favour of leaving the United Kingdom and joining the Irish Republic.

Tory MPs are discussing the issue, with Brexiteers furiously rejecting the findings and insisting that any future border poll on a united Ireland would be winnable. One European Research Group (ERG) source said Mrs May’s insistence that the union may be