The EU's chief negotiator Michel Barnier on Thursday hailed the last-minute Brexit deal struck with Britain ahead of a crunch summit as a reasonable compromise that protects European interests.

"We have reached an agreement that is fair and reasonable and corresponds to our principles," Barnier told reporters, but urged caution as the deal must still be passed by the British parliament -- which has rejected a Brexit deal three times before.

Prime Minister Boris Johnson will take the accord to the House of Commons on Saturday and it is far from certain it will be approved, with his allies the small Northern Irish Democratic Unionist Party (DUP) rejecting the compromise before it was even announced.

Weeks of tense negotiations between Barnier's team and British officials resulted in tweaks to the arrangements for Northern Ireland to modify the so-called "backstop" that had enraged the DUP and hardline Brexit supporters.

The final agreement sees Northern Ireland aligned to a limited number of EU rules, notably to do with goods, but remain in Britain's customs territory -- meaning it can benefit from future trade deals London might strike with other countries.

To address the vital question of democratic consent, Northern Ireland's regional assembly will vote every four years on whether to maintain the arrangements.

If the assembly votes them down, Barnier explained, there will be a "cooling-off period" of two years to work out what to replace them with.

Johnson spoke with European Commission chief Jean-Claude Juncker by phone on Thursday morning.

Barnier said Johnson had told Juncker that he was "confident of convincing a majority of lawmakers in the House of Commons". (AFP)