This article originally published in October has been updated to take account of the controversy surrounding the prime minister’s latest comments.

Boris Johnson has repeated claims that there will be “no checks” between Northern Ireland and Britain under his new Brexit deal – despite evidence to the contrary from trade experts, official documents, and the text of the divorce deal itself.

His comments are important because arrangements for Northern Ireland go to the heart of the Brexit debate – and are crucial to future EU-UK trade and arguably to the territory’s place in the United Kingdom.

They are also fundamental to the question of trust in the prime minister, which has emerged as one of the key themes in the election campaign. Such is Boris Johnson’s reputation that when he claimed during a TV debate that truth mattered, the audience laughed.

A reminder – what does the revised Brexit deal say?

The new deal enabled the prime minister to get the controversial backstop guarantee struck out of the EU withdrawal agreement. This switched the focus from the future UK-EU border between Northern Ireland and the Irish Republic, to the internal UK relationship between Northern Ireland and Great Britain.

Under the new protocol in the UK-EU divorce deal, Northern Ireland would leave the EU’s customs union with the rest of the UK, which wants to pursue an independent trade policy. But, in practice, it would follow EU customs rules and remain aligned with some aspects of the EU’s single market.

Unionists are furious at proposals they allege weaken the territory’s position as part of the United Kingdom, creating a “border down the Irish Sea”. The question of possible extra checks and controls within the UK is highly sensitive.