The Government here expects the UK's Brexit transition to extend beyond the two-year time line suggested by British Prime Minister Theresa May, according to Bloomberg which cites a person familiar with the matter.

The so-called implementation period after the UK exiting the European Union laid out by Mrs May in her Florence speech last week may take as as long as five years, according to the person, who is familiar with the Government’s preparations for Brexit and asked not to be named as the deliberations are private.

During this period, Britain’s future relationship and trade terms with the EU will have to be worked out, approved by member states and then put into place.

"If they want a transition period to serve its purpose, the final deal itself would have to be completely agreed upon and signed off in advance," said Katy Hayward, a political sociologist at Queen’s University Belfast.

"And if they want a bespoke deal, something between the Norway and Canada model, that would mean that two years’ transition is very unlikely to be sufficient time.

"Ireland, the EU economy most vulnerable to Brexit, is seeking as long a transition as possible, Minister for Foreign Affairs Simon Coveney said yesterday.

The EU has made clear that any transition should be time-limited, and the pro-Brexit members of Mrs May’s government were reluctant to accept even a two-year bridging deal.

Keeping the border between Northern Ireland and the Republic open after Brexit is one of three divorcetopics, along with citizens’ rights and money owed by the UK, that require "sufficient progress" before the EU will allow talks to move on to the future relationship.

At October’s summit of EU leaders, it’s not likely enough movement will have been made to allow talks to shift gear, according to the person.

While the issue of citizens’ rights is close to resolution, both sides remain divided on Britain’s bill, the person said.

Progress has been made on the Irish issue, though a final resolution will be parked until the shape of the UK’s future relationship is clearer, the person said.

Mrs May said on Friday there would be no physical infrastructure on the border after Brexit, a declaration welcomed by Taoiseach Leo Varadkar.

Last month, the Government said the UK paper on Ireland represented significant steps forward, though more clarity was needed.

In particular, officials here see the UK’s proposal for a so-called trusted-trader programme as a realistic way to keep trade flowing across the border after the split.

Cross-border trade is worth more than €3 billion a year, the State estimates.

The UK government had no comment beyond Mrs May’s Florence speech, while the Department of Foreign Affairs was unable to comment.