The European Union is ready to start negotiations on Britain's exit from the bloc immediately, despite London's decision to delay triggering the divorce process, the Union's president Donald Tusk has said.

Key points: EU leaders meet without Britain for the first time post-Brexit

EU leaders meet without Britain for the first time post-Brexit Summit agreed on "road map" for EU reform

Summit agreed on "road map" for EU reform President Donald Tusk says Brexit negotiations "could begin tomorrow"

Speaking at a summit in Bratislava of the 27 EU leaders without Britain, Mr Tusk said that the Union was "well prepared" and added that "we could even start the procedure tomorrow".

German Chancellor Angela Merkel and French President Francois Hollande vowed on Friday to make the European Union a "success", in a show of unity after a crucial summit to plot the bloc's post-Brexit future.

The joint statement from the continent's traditional axis of power came after the EU's 27 leaders, minus Britain, gathered in a castle overlooking the Slovak capital Bratislava to respond to multiple challenges.

Despite deep divisions across the bloc, the leaders agreed on a "roadmap" for migration, terrorism, defence and the economy which they aim to approve in Rome in March to mark the 60th anniversary of the EU's founding treaties.

"Germany and France will very intensively play their part in the coming months in order to make all this a success," Ms Merkel said.

EU situation 'critical' post-Brexit vote

Ms Merkel repeated her warning that the EU was in a "critical situation", but her influence as leader of the EU's biggest economy has been undermined by her unpopular decision to open Germany's doors last year to nearly 1 million refugees.

Mr Hollande, who leads Europe's top military power now that Britain is on the way out, said that France and Germany would "continue to work so that we can deliver concrete measures".

France and Germany produced a joint defence plan in the run-up to the summit, in a sign that the EU's 'big two' wanted to take the lead in rebuilding confidence in the EU following Britain's shock June 23 decision to leave.

The "informal" summit — so-called because any formal meeting still has to include Britain until it leaves the bloc — is aimed at restoring public faith in the EU, which for decades was seen as a guarantor of peace and prosperity but is now, officials acknowledge, in an "existential crisis".

Bloc faces multiple challenges

Mr Tusk has said he hoped for a public show of unity after Britain's vote to leave; wanting to restore EU stability and credibility to counteract the migrant and euro-crises.

Years of economic crisis have pushed up unemployment in many member states, while a spate of attacks by Islamist militants and a record influx of migrants have unsettled voters, who are turning increasingly to populist, anti-EU parties.

"All these countries have an interest in fighting terrorism, in a more secure Europe, in preventing uncontrolled migration, all have an interest in economic cooperation to create more jobs," Dutch Prime Minister Mark Rutte said.

Ahead of the meeting, Mr Hollande said all members were aware of the challenges the bloc was facing.

"Everyone is aware of the situation. Britain has decided to leave and there are questions about the future of Europe," he said.

"Either we move in the direction of disintegration, of dilution, or we work together to inject new momentum, we relaunch the European project."

Greek Prime Minister Alexis Tsipras said Europe needed to "stop sleep-walking in the wrong direction".

Any reform may not be possible until late 2017 when the Netherlands, France, and Germany have held elections, and even then it is unclear whether France and Germany can bridge difference over economic policy.

AFP/Reuters