BREXIT talks between the UK and EU have cleared a key stage on the road to a deal, with diplomats now entering an intensive, secret final phase known as the “tunnel”.

It follows "constructive" meetings between Boris Johnson and the Irish PM Leo Varadkar on Thursday, and between Brexit Secretary Stephen Barclay and Michel Barnier today.

The development means EU member states consider there is merit in accelerating talks ahead of the crunch EU council meeting next week.

However, it also implies a major UK concession over customs and consent arrangements in Ireland that could upset the DUP and make any deal unlikely to pass at Westminster.

It may also reflect the UK and EU trying to keep the talks alive artificially rather than be the side that admits they have failed in order to avoid blame. 

EU Council President Donald Tusk last week warned Mr Johnson that unless there was progress towards a workable alternative to the Irish backstop, talks would effectively end today and there would be no advance to the tunnel stage.

It was reported that EU leaders had given the green light to talks more after Mr Barnier, the EU’s chief negotiator, reported he had had a “constructive meeting” with Mr Barclay.

However both sides were restrained about the ultimate prospects for success.

Mr Barnier said: “I’ve already said that Brexit is like climbing a mountain: we need vigilance, determination and patience.”

READ MORE: Michel Barnier hails "constructive meeting" with Brexit Secretary Stephen Barclay

The progress follows an unexpectedly upbeat assessment of the situation from the Taoiseach after his meeting with Boris Johnson on Thursday.

After referring to his “grave difficulty” with the UK’s proposed alternative to the Irish backstop earlier in the week, Leo Varadkar said he and Mr Johnson could see a pathway to a deal.

However, he has refused to say what has changed.

“I think at this stage probably the less said the better,” he said in Dublin today.

Mr Tusk was also giving little away.

He said:“Prime Minister Johnson promised the EU to come forward with a solution that would work for all. 

“A solution that would not only satisfy the hardcore Brexiters but also solve our well-known and legitimate objectives: to avoid a hard border on the island of Ireland, to protect the Good Friday agreement, and ensure the integrity of the single market.

“Unfortunately we are still in a situation in which the UK has not come forward with a workable realistic proposal.”

READ MORE: Brexit deal on brink of collapse after Johnson-Merkel call

But he went on: “However, yesterday, when the Irish taoiseach and the UK prime minister met they both saw for the first time a pathway to a deal. I have received promising signals from the taoiseach that a deal is still possible.”

“Of course, there is no guarantee of success and the time is practically up, but even the slightest chance must be used. A no-deal Brexit will never be the choice of the EU.”