Brexit 'won't be quick or painless'

It is an "illusion" to think that the process of Britain leaving the European Union will be quick or painless, the European Commission's chief negotiator Michel Barnier has warned.

Barnier was speaking as he set out the Commission's detailed negotiating guidelines for the first phase of withdrawal talks, expected to begin in earnest after the June 8 general election.

His comments came as news reports suggested that the Commission may present the UK with a 100 billion euro ($A146 billion) "divorce bill" had sharply increased from the previous estimates of 60 billion euros.

But Brexit Secretary David Davis rejected suggestions that the UK will foot a bill on that scale, saying Brussels will only receive what it is legally owed.

Mr Davis also dismissed as "laughable" reports that Prime Minister Theresa May would be barred from negotiating with her counterparts.

In a press conference in Brussels, Mr Barnier said that the first phase of negotiations would focus on the financial settlement, the status of expatriate citizens and the future border between the UK and EU in Ireland.

He insisted that the the so-called divorce bill was not a punishment of the UK for leaving, but simply a "settling of accounts".

"The UK must put a great deal of energy and effort into these three issues over the next weeks and months and that will increase the chances of making a deal," said Mr Barnier.

"Some have created the illusion that Brexit would have no material impact on our lives or that negotiations can be concluded quickly and painlessly. This is not the case.

"We need sound solutions, we need legal precision and this will take time."