European leaders may need to hold an extra summit in November to clinch a Brexit deal and avert ‘a catastrophe’ when Britain leaves in March, EU President Donald Tusk warned Tuesday.

When European Union leaders meet this week in the Austrian Alps, they will discuss how to clear both the last hurdles to a divorce deal and to a plan for future ties, he said.

‘We will discuss how to organise the final phase of the Brexit talks, including the possibility of calling another European Council in November,’ he said, in a letter inviting the 28 leaders to the informal summit.

The leaders who make up Tusk's European Council had been aiming to strike a Brexit deal at a scheduled October summit, but now seem to be on course to hold another last-ditch get-together in November.

This would still give the British parliament and those in European member states enough time to debate and ratify the terms before the end of March, when Britain would otherwise crash out without a plan.

‘If we all act responsibly, we can avoid a catastrophe,’ Tusk wrote, warning of the fallout from no deal.

- Salzburg summit -

He stressed the need to find ways to ensure ‘there will be no hard border in the future’ between Ireland and British Northern Ireland, which Brussels sees as key to preserving the peace agreement.

EU officials and many British experts are also worried that a no-deal Brexit would damage economic and financial ties between the world's biggest trading bloc and one of the world's largest economies.

Chancellor Sebastian Kurz of Austria -- which currently holds the bloc's six-month rotating presidency -- will open the two-day summit on Wednesday evening in Salzburg.

He will host a dinner where British Prime Minister Theresa May briefs her EU counterparts about the state of talks between her negotiator Dominic Raab and Europe's Brexit pointman Michel Barnier.

Tusk said the 27 other leaders would meet on Thursday without May to discuss a statement of intent on future relations with Britain, which is to be announced along with the Brexit divorce deal.

- 'Level playing field' -

EU diplomats warn that British demands for partial single-market access could undermine ‘the level playing field’ if Britain refuses to stick to EU health, labour and environment standards and seeks to make exports like steel cheaper.

Producing a possible new wrinkle in talks, a British government-commissioned report said Britain should not offer preferential access to EU workers over those from outside the bloc after Brexit.

One European diplomat told AFP he still expects May to seal a deal even though her own ‘Chequers’ blueprint crosses European red lines and has angered hardline Brexiteers in her own Conservative party.

But the diplomat expected little headway in Salzburg as both sides act cautiously before the Conservative Party conference on September 30.

The EU is standing by its demand for a legal backstop on Ireland.

It has proposed that Northern Ireland stay aligned with the remaining 27 bloc members after Brexit as part of a ‘backstop’, or insurance policy to avoid the reimposition of border checks.

- Japan urges clarity -

But Britain, fearing the EU proposal would break up its territory, has suggested instead that the whole country remain aligned with the EU in certain areas, but only until the end of 2021.

Deadlock on the Irish border has fuelled concerns Britain may leave without securing a deal, cripple its economic and finance sectors and create food shortages and giant traffic jams near ports.

British police chiefs said Tuesday they were preparing for a no-deal scenario but warned that losing access to dozens of EU-wide tools will make it ‘harder’ to protect UK and European citizens.

In Tokyo, Japan's foreign minister on Tuesday urged his visiting British counterpart to ensure clarity over Brexit amid signs Japanese businesses are growing increasingly nervous about Britain's future outside the EU.

In August, May played down the consequences of a no-deal when she quoted World Trade Organization director general Roberto Azevedo as saying it would ‘not be the end of the world.’

But the International Monetary Fund warned on Monday that such a scenario would inflict ‘substantial costs’ on the British economy.

Aside from the travails of Brexit, the bulk of the summit will tackle migration and security.