A deal to smooth Britain's departure from the European Union hangs in the balance after diplomats indicated the bloc wanted more concessions from British Prime Minister Boris Johnson and said a full agreement was unlikely this week.

As the Brexit maelstrom spins, Mr Johnson and EU leaders face a tumultuous week of reckoning that could decide whether the divorce is orderly, acrimonious or delayed yet again.

Mr Johnson says he wants to strike an exit deal at an EU summi ton Thursday and Friday to allow an orderly departure on 31 October.

If an agreement is not possible, he says he will lead the UK out of the bloc it joined in 1973 without a deal – even though parliament has passed a law saying he cannot do so.

However current EU president Finland said more time was needed and that negotiations could continue even after the EU summit.

"I think there is no time in a practical or legal way to find an agreement before the EU Council meeting," Prime Minister Antti Rinne said after talks with the next chair of EU summits, Charles Michel.

"We need more time and we need to have negotiations after the (European) Council meeting."

Some EU politicians, such as Tánaiste Simon Coveney, said a deal was possible with more work.

However EU diplomats were pessimistic about the chances of Mr Johnson's hybrid customs proposal for the Irish border riddle.

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"We are not very optimistic," a senior EU diplomat told Reuters.

After more than three years of Brexit crisis that has claimed the scalps of two British prime ministers, Mr Johnson will have to ratify any last-minute deal in parliament, which will hold its first Saturday sitting since the 1982 Falklands War.

As EU ministers met in Luxembourg ahead of the leaders' summit, Mr Johnson's planned legislative agenda was read out by Queen Elizabeth at the state opening of parliament.

"Let's not wait - we can't wait: let's get Brexit done," Mr Johnson told parliament.

"If there could be one thing more divisive, more toxic than the first referendum, it be would be a second referendum."

The pound fell more than 1% to a session low of $1.2517. Against the euro, the British currency weakened by a similar margin to 88.11 pence.