Boris Johnson said Britain was ready to leave EU without a deal on October 31. (Reuters)

Britain on Wednesday unveiled its own Brexit plan for keeping the Irish border open, with Prime Minister Boris Johnson warning the only alternative was leaving the European Union on October 31 without any deal.

After earlier saying that this was his "final" offer made 29 days before the Brexit deadline, Johnson wrote in a letter to European Commission president Jean-Claude Juncker that London's long-awaited proposal was "a reasonable compromise".

Central to the plan is the removal of the so-called "backstop" to prevent the return of customs posts between EU member Ireland and the British province of Northern Ireland once the UK ends its 46-year involvement in the European project.

That plan would effectively keep Britain in a temporary customs union with the EU, which Brexit-backing critics argued would force London to abide by the bloc's rules indefinitely and nullify the 2016 EU referendum's results.

Johnson told Juncker that the EU plan was "a bridge to nowhere". "A new way forward must be found," he wrote.

After a phone call later on Wednesday, Juncker warned Johnson there were "problematic points" in Britain's new proposal, but said EU negotiators would "examine the legal text objectively".

Irish premier Leo Varadkar had also called overnight leaks of the new proposal "not promising".

At the heart of the proposals is honouring the terms of the 1998 Good Friday Agreement, which brought an end to three decades of sectarian violence in Northern Ireland.

That historic pact was based on the premise of an open frontier with Ireland -- a principle that Johnson's plan said Britain was "absolutely committed to upholding".

In place of customs checks at the border, Britain's plan proposes that "goods movements between Northern Ireland and Ireland will be notified using a declaration".

Physical checks would instead be conducted at traders' premises or specific, designation locations but not at Ireland's border with British Northern Ireland.

Ireland has strongly opposed the reintroduction of border customs checks, calling them a dangerous approach.

In place of customs checks and once a post-Brexit transition period ends, the UK plan would create "an all-island regulatory zone on the island of Ireland" that covers "all goods" -- not just agriculture and food products mentioned in earlier reported drafts of the plan.

This regulatory zone, which would see Northern Ireland temporarily follow EU standards and regulations, would be "dependent on the consent of those who live under it, through the Northern Ireland institutions," the plan says.

Crucially, Johnson's plans allows a new Northern Irish assembly to decide whether to extend this arrangement once "every four years", putting a potential time limit on the plan that the EU has previously rejected.

Call for compromise

Johnson had earlier told members of his ruling Conservative party that Britain was prepared to leave the EU without a deal, despite fears it could herald an economic slump.

Northern Ireland's Democratic Unionist Party, whose lawmakers support Johnson's minority government in Britain's parliament, broadly welcomed the plan.

"This offer provides a basis for the EU to continue in a serious and sustained engagement with the UK Government without risk to the internal market of the United Kingdom," it said.

London's benchmark FTSE 100 index -- already falling heavily on weak global growth concerns -- fell 3.2 percent, indicating investor concerns about the plan's prospects in Brussels.

The Confederation of British Industry has said a no-deal Brexit would be "a historic failure of statecraft" which would dog growth and trade "for years to come".

Hostile parliament

Johnson, a leading "leave" campaigner in the 2016 EU referendum, took office in July vowing to deliver Brexit at the end of this month at all costs.

But like his predecessor Theresa May, he has struggled against a hostile parliament and the complexities of untangling four decades of EU integration.

His promise to leave without a deal was derailed when MPs last month passed a law demanding he seek to delay Brexit if he has not reached an agreement by an EU summit on October 17.

Johnson is now racing to renegotiate the exit terms May agreed with Brussels, which were rejected by the British parliament three times.

May was force to twice delay Brexit as she tried and failed to push her deal through the House of Commons, resulting in her resignation in May.

Johnson has also faced significant opposition among MPs and lost his wafer-thin Commons majority during a rebellion over his EU strategy earlier this month.

Yet even after being slapped down last week by Britain's Supreme Court for unlawfully trying to suspend parliament, Johnson still insists he will never ask for a delay.