European Council President Donald Tusk has warned Britain that its plan to leave the EU's customs union and single market on Brexit could mean a return to a "hard border" on the island of Ireland.

Addressing a business conference in Brussels before leaving for lunch in London with British Prime Minister Theresa May, the EU summit chair said an EU proposal yesterday to incorporate Northern Ireland within a "common regulatory area" with the EU was the best option to avoid border friction - but he would be asking in London if Britain could propose something better.

"Until now, no one has come up with anything wiser than that," Mr Tusk told the Business Europe event.

"In a few hours, I will be asking London whether the UK government has another idea that will be as effective in preventing a hard border between Ireland and Northern Ireland."

Mr Tusk also confirmed that he will distribute negotiating proposals next week for a future trade relationship with Britain. That will follow May's expected announcement of her proposals tomorrow.

But, Mr Tusk warned, Mrs May's "red lines" of leaving the single market and customs union meant that some friction in EU-UK trade would be inevitable.

"There can be no frictionless trade outside of the customs union and the single market. Friction is an inevitable side effect of Brexit, by nature," he said.

Mrs May briefed her cabinet this morning on the speech she will deliver tomorrow.

With both Brussels and Dublin saying they are willing to look at alternatives to the backstop option, in other words the common regulatory area, observers will be looking closely for detail that will be both politcally and legally acceptable to the EU, given its determination to protect the integrity of the single market and customs union.

In a speech this afternoon in Brussels, the former British prime minister Tony Blair will say that if Britain changed its mind on Brexit, then the EU should come forward with reforms to the freedom of movement rules that the UK has historically been resistant to.

Meanwhile, Taoiseach Leo Varadkar has said he will be listening closely to Mrs May's Brexit speech tomorrow.

He said he hoped to hear some clearer detail as we are now well beyond good intentions and aspirations.

He said a withdrawal agreement had now been produced with an Irish protocol which writes down in black and white how we can avoid a hard border on the island of Ireland.

Mr Varadkar said he would welcome proposals from the UK that "wouldn't force us to use this backstop arrangement."

He said he wanted a close trading relationship with all of the UK and Europe.