The European Commission has released an outline of five possible futures for the EU after Brexit - including one scenario where Brussels dramatically scales back its activities to focus on the single market only.
The paper makes no recommendation of which route Europe should follow, though Commission president Jean-Claude Juncker said he personally rejected the option of reducing the EU to a free trade area.
In a warning to the 27 remaining states against following the UK's lead in going it alone, the White Paper on the Future of Europe cautions against "the allure of isolation", predicting that a break-up of the EU would leave individual states "prey to the interests of stronger powers".
Former UK deputy prime minister Nick Clegg - now EU spokesman for the Liberal Democrats - said the document showed the EU was "serious about the need for reform".
The Birtish government's pursuit of a "hard Brexit" would leave Britain's voice "irrelevant" in the future development of the continent, he said.
The paper was released in Brussels ahead of discussions on the way ahead at the EU's 60th anniversary summit in Rome on March 25.
The gathering will be attended by leaders of 27 member states but not by Prime Minister Theresa May, who is due to trigger Britain's withdrawal negotiations by the end of the month.
Unveiling the document to the European Parliament, Mr Juncker said: "However painful or regrettable Brexit may be, it will not stop the European Union as it moves to the future. We need to move forward, we need to continue."
The five scenarios set out in the paper include:
Carrying On - The EU sticks to its course, implementing and upgrading its current reform agenda.
Nothing But The Single Market - The EU focuses on deepening the single market and significantly cuts regulation, while scaling back co-operation on issues like migration, security or defence.
Those Who Want More Do More - The EU allows member states to work together in several "coalitions of the willing" on specific policy areas, ranging from defence or internal security to taxation or social affairs.
Doing Less More Efficiently - The EU decides to focus its attention and resources on a reduced number of areas, such as innovation, trade, security, migration, borders and defence.
Doing Much More Together - Member states decide to share more power, resources and decision-making across the board.
The document acknowledges that many European citizens regard the EU as "too distant or too interfering in their day-to-day lives" and question whether it improves their standard of living. And it says the challenges of financial crisis, refugees, terrorism and Brexit "show no signs of abating".
But it warns: "In an uncertain world, the allure of isolation may be tempting to some, but the consequences of division and fragmentation would be far-reaching. It would expose European countries and citizens to the spectre of their divided past and make them prey to the interests of stronger powers.
"Europe must now choose. There are as many opportunities as there are challenges. This can be Europe's hour, but it can only be seized by all 27 member states acting together with common resolve."
Mr Juncker said: "Sixty years ago, Europe's founding fathers chose to unite the continent with the force of the law rather than with armed forces.
"We can be proud of what we have achieved since then. Our darkest day in 2017 will still be far brighter than any spent by our forefathers on the battlefield.
"As we mark the 60th anniversary of the Treaties of Rome, it is time for a united Europe of 27 to shape a vision for its future. It's time for leadership, unity and common resolve.
"The Commission's White Paper presents a series of different paths this united EU at 27 could choose to follow. It is the start of the process, not the end, and I hope that now an honest and wide-ranging debate will take place. We have Europe's future in our own hands."
Mr Clegg said: "This paper shows that, contrary to the claims of the Brexiteers, the EU is serious about the need for reform. Theresa May's pursuit of a hard Brexit will make Britain's voice even more irrelevant as wider changes unfold in Europe."
The leader of Conservative MEPs, Ashley Fox, said: "I am pleased to see president Juncker laying out a series of options for the future of the EU rather than once again simply pushing his tired vision of a federal Europe.
"It is a little late, but he is finally acknowledging there is more than one way for the EU to proceed."
A Downing Street spokesman said: "We have been very clear that we want the EU to be a success, for the 27 states to continue to thrive together. That is not only in their interests, but it is in our interests as well."