BRUSSELS (Reuters) - Dominic Raab tripped up on Michel Barnier’s Brexit staircase -- that is how EU negotiators see how the reworking of a draft deal reached a month ago led to new demands on Britain that prompted him and other ministers to resign.

FILE PHOTO: Britain's Secretary of State for Exiting the European Union, Dominic Raab delivers his speech outlining the government's plans for a no-deal Brexit in London, Britain. Aug 23, 2018. REUTERS/Peter Nicholls/File Photo

The "staircase" in question is a somewhat crude infographic produced by the European Union's chief negotiator a year ago -- it can be viewed -- which shows different types of trade ties that countries can have with the bloc.

It lays out a descending trade-off between rights and obligations, from full EU membership at the top left (many rights like full access to the EU market but many obligations such as open migration, strict EU rules and budget payments) to Canada’s free trade pact EU at bottom right, which commit the North Americans to relatively little beyond the commercial area.

According to EU officials and diplomats close to the Brexit negotiations, it was demands by Brexit Secretary Raab and Prime Minister Theresa May to seek more rights than were offered in a draft deal a month ago -- notably a guarantee of a customs union -- that led to Wednesday’s deal ending up laden with further obligations which Raab and other Brexiteers could not stomach.

“They came up against Barnier’s staircase,” one senior EU official said of how the drafts were transformed since Oct. 13 to build in more rights for Britain but also more demands. “We have to have an exact balance of rights and obligations.”

That, EU diplomats say, is also something that Britons who want to reopen negotiations would do well to bear in mind.

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MONTH OF CHANGE

On Oct. 13, the technical experts hammering out the details of the pact -- Barnier’s deputy Sabine Weyand and May’s adviser Oliver Robbins -- arrived at a draft deal that addressed fears of disruptive checks on the Irish land border by promising that Britain could negotiate an EU customs union.

This was a crucial EU concession, as Barnier noted after a new deal was struck on Wednesday, because it overrode an EU demand that Britain’s province of Northern Ireland could be in a separate customs zone from the mainland if hopes for other trade accords that would keep the border open failed to materialise.

However, that Weyand-Robbins deal failed to pass muster in London and when Raab came to Brussels to kill it off on Oct. 14 he made clear that Britain needed much more of a guarantee that it would get a customs union deal as an “all-UK backstop”.

The previous draft included only a commitment to work on a single customs area in the future, but no details of what that would look like. And in the event it was not agreed, Northern Ireland would have to stay in a customs union with the EU, with or without the rest of the UK.

“That was not enough for the UK government because there was no cast-iron guarantee there would not be a customs border between Northern Ireland and Great Britain,” a senior EU official said on Thursday.

The negotiators went back to the drawing board. The EU had been telling London for months that it could not extend sweet terms offered to struggling Northern Ireland alone -- giving it access to EU markets -- to Europe’s second biggest economy.

But, they said, if Britain accepted more EU regulation of its industries in order to counter complaints from other EU governments that it was getting a free pass, it might work.

The result was scores more pages added to the draft treaty aimed at ensuring a “level playing field” for France, Germany and the others over British industries getting custom-free access to the EU market. For good measure, and French favour, they also added demands on access to Britain’s fishing grounds.

May called it the best deal available. It proved, however, to be a step too far for several of her ministers, including Raab. He resigned saying: “No democratic nation has ever signed up to be bound by such an extensive regime, imposed externally without any democratic control over the laws to be applied.”

While there is speculation in London of a challenge to May and of other Britons coming back to Brussels to renegotiate, EU diplomats and officials say they could imagine yet more talks. But those EU trade-offs between rights and obligations will not change, they say, and Britons must beware Barnier’s staircase.