EU President Donald Tusk said on Tuesday that the outcome of fraught Brexit talks was ‘up to London’ and that abandoning the EU divorce remained an option for the UK.

Brussels has stressed that Britain needs to offer more detailed proposals, particularly on the thorny issue of its divorce bill, if it wants to move talks on to the next phase, on Britain's future relationship with the bloc.

Tusk told the European Parliament that the European Union must stay united in talks or face ‘defeat’, as British Prime Minister Theresa May struggles to contain divisions within her government and businesses grow increasingly worried about a so-called ‘hard Brexit’ -- in which the UK crashes out of the bloc in March 2019 with no trade deal in place.

‘The EU will be able to rise to every scenario as long as we are not divided,’ Tusk told MEPs in the eastern French city of Strasbourg.

‘It is in fact up to London how this will end: with a good deal, no deal or no Brexit,’ he said, reiterating the controversial idea that the EU was open to Britain backtracking on its decision to leave the bloc.

European leaders gave the embattled May a helping hand at a summit on Friday by agreeing to start preparations for the next round of talks while stressing that not enough progress had been made on three key initial areas to move on just yet.

EU officials say May's offer in a speech in Florence last month to maintain Britain's contributions for two years after Brexit to complete the current EU budget period, totalling around 20 billion euros ($24 billion) was not enough.

Fearing that Britain's departure in 2019 will leave a gaping hole in the EU budget, European capitals are demanding detailed commitments -- rather than just rhetorical gestures -- before agreeing to start trade talks.

French President Emmanuel Macron warned London on Friday that agreement on the financial settlement was still ‘a long way off’.

As well as the financial settlement, the EU wants progress on the rights of three million European citizens living in Britain and the issue of the Irish border.

- Unity or 'defeat' -

Ahead of last week's summit May engaged in a frantic campaign of diplomacy to try to win over key EU leaders away from the official negotiating structure, which Brussels is running through the European Commission.

Former Polish PM Tusk told MEPs that any flaw in the bloc's united front would end in disaster.

‘We have managed to build and maintain unity among the 27 (remaining EU members), but ahead of us is still the toughest stress test,’ he said. ‘If we fail it, the negotiations will end in our defeat.’

With May under intense pressure at home, the atmosphere around the talks is becoming increasingly fraught, a fact highlighted by a German press report Sunday saying she had ‘begged’ commission chief Jean-Claude Juncker for help during a pre-summit dinner last week.

Juncker hotly denied the report and on Tuesday he strongly underlined that the commission, which handles the talks for the EU, was approaching the negotiations in good faith.

‘The commission is not negotiating in a hostile mood,’ he told the European Parliament.

‘We want a deal. Those who don't want a deal, the no dealers, they have no friends in the commission,’ said Juncker.

On Monday Dutch electronics giant Philips became the latest major business to warn over the effects of Brexit, with chief executive Frans van Houten telling AFP demand was falling in the UK.