European Council President Donald Tusk stressed the importance of preserving unity among the 27 remainingEuropean Union member states involved in Brexit negotiations.
Mr Tusk described the negotiations are the EU's "toughest stress test."
Addressing the European Parliament in Strasbourg following last week's European Council meeting, Mr Tusk said, "It is up to London how this will end - with a good deal, no deal or no Brexit".
He warned that Brexit is "still the toughest stress test. If we fail, the negotiations will end in our defeat."
The former Polish prime minister stressed the importance of the 27 member states maintaining unity and claimed that the EU can rise to any scenario as long as it is not divided.
Meanwhile EU negotiator Michel Barnier has said Britain must accept the full economic and legal status quo in a transition period after it leaves the EU.
Mr Barnier said Britain should expect no tailor-made terms on trade in its future relationship.
Suggesting a transition should run for 21 months from Brexit on 30 March 2019 until the current EU budget expires at the end of 2020, Mr Barnier said Britain would "certainly" remain subject to EU laws and courts during that transition.
"During this period, the EU legal framework including on jurisdiction would continue to apply to Britain," Mr Barnier said in an interview published in Germany's Handelsblatt newspaper.
"We don't have time to invent a new model. So for a short time after the formal exit from the EU the economic status quo would continue to apply, which besides the internal market also includes the customs union and collective political decisions."
In a version of the interview in Belgium's L'Echo, he said of the transition: "The only difference is that the British would no longer take part in decisions on European legislation."
British Prime Minister Theresa May has proposed a transition of around two years to give time to put a new free trade pact in place, although she faces opposition in London from some Brexit supporters who want a clean, quick break.
Mr Barnier noted that Mrs May had rejected the option of staying in the EU single market long-term - the "Norwegian model" as he called it, referring to Norway's membership of the EU internal market, accepting all its rules and costs without having a say.
"So we must work on other hypotheses," Handelsblatt quoted him as saying.
"Another option would be a free trade treaty using the example of the CETA agreement with Canada. It would take several years, however, to negotiate such an agreement."
Asked if that meant there could be "a specifically British model" along the lines of the "bespoke arrangements" Mrs May has referred to without giving detail, Mr Barnier replied simply: "No."
Quoted by L'Echo, he said a trade deal could be agreed in three years - meaning that if talks start in December it would be ready just in time for a transition ending in December 2020.
He said his staff were already working on drafting a withdrawal treaty that will include terms for transition.
Mr Barnier said he still hoped that Mrs May could provide more detail on her offer to meet financial commitments on withdrawal so that EU leaders could agree in December to end their refusal to negotiate a future trade deal.
Mrs May told an EU summit last week that she could not agree a figure until she knows what trade terms the EU offers.
Mr Barnier said it was important to "de-dramatise" talks on the payment.