LONDON - Former British Foreign Secretary Boris Johnson warned Monday that continuing to press ahead with Prime Minister Theresa May's Brexit plan will end in a spectacular political car crash.

Johnson said if the so-called Chequers plan for Britain leaving the European Union is utterly unacceptable in principle, the practical consequences are even worse.

He made his warning in the Daily Telegraph newspaper as it was revealed May has issued a stark warning that it will be her deal or no deal.

Johnson's weekly columns have become essential reading for people on both sides of the leave and remain sides.

In his column Monday he zooms in on the problem of the post-Brexit border between Northern Ireland and the Irish Republic which are separated by a 500 km land border.

In March the EU commission published a protocol spelling it out: that until Brussels agreed otherwise, Northern Ireland would have to remain effectively part of the EU.

As it stands, said Johnson, an EU version of resolving the border issue "is little short of an attempt to annex Northern Ireland".

"It would imply customs and regulatory controls between Britain and Northern Ireland, and therefore a border down the Irish sea," he claimed.

The suggested protocol, said Johnson, would amount to a change in Northern Ireland's constitutional status and a total breach of the peace agreement that ended decades of troubles, and thousands of killings, in Northern Ireland.

"We are now saying that if Brussels cannot be satisfied on our plans for the Irish border, then we are volunteering that the whole of the UK must remain effectively in the customs union and large parts of the single market until Brussels says otherwise," claimed Johnson.

In a major television interview to be aired Monday night local time to mark six months to go before Britain leaves the bloc, May has insisted it will be her deal or no deal.

Speaking in the interview to the BBC, May says that if the British parliament does not ratify her Chequers plan "I think that the alternative to that will be having no deal".

May adds there needs to be friction-free movement of goods with no customs or regulatory checks between Britain and EU on the island of Ireland, in order to avoid a hard border on the island of Ireland.

Meanwhile the Times newspaper in London claimed Monday that the EU is "secretly preparing to accept a frictionless Irish border".

The newspaper reports that the EU's chief negotiator Michel Barnier is working on a new protocol text outlining a plan to use technology to minimise border checks.