Sixty years ago, Britain shunned a meeting in Rome where six war-scarred neighbours founded what became the EU; on Saturday, it is again absent, this time from a sombre birthday party as it quits a bloc which now embraces most of Europe.

It might have been a modestly hopeful summit to mark the 28-nation European Union's 60th anniversary in the palazzo where old foes France and Germany, with Italy and the Benelux countries, signed the Treaty of Rome on March 25, 1957.

Instead, it will be overshadowed by the unprecedented departure of a member state, which Prime Minister Theresa May will initiate next Wednesday.

All the bloc's economies are growing after a slump that has blighted the past decade and recent border chaos has largely abated as refugees are, for now, being held in check.

But Brexit, which should take effect in March 2019 if a two-year timetable holds, has undermined the self-confidence of a Union that might see its 60 years of strengthening peace and growing prosperity as a success, and has encouraged eurosceptic nationalists challenging governments from Stockholm to Sicily.

It has also amplified the petty frictions among the more than two dozen national governments and obliged leaders' aides to water down a grand birthday declaration of unity.

Britain originally snubbed the Treaty of Rome, but changed its mind about the Common Market three years later. It was made to wait however by the suspicious French until 1973 before joining and it voted to leave the Union last June in a divisive referendum whose consequences remain unclear.

After days of carping from Poland and Greece, seeking to show home voters they were getting Brussels to give assurances about equal treatment and social welfare, the Rome Declaration the 27 will sign in the late morning offers ringing phrases about peace and unity.

But it may disappoint those who think more ambition and coordination is the answer to malaise.

PAPAL MESSAGE

At the Vatican on Friday, Pope Francis told the assembled leaders, Roman Catholics and non-Catholics alike, that their Union had achieved much in 60 years but that Europe faced a "vacuum of values". He condemned anti-immigrant populism and extremism that he said posed a mortal threat to the bloc.

"When a body loses its sense of direction and is no longer able to look ahead, it experiences a regression and, in the long run, risks dying," said the Argentinian pontiff.

Their response, he said, should be to promote Europe's ideals and values with more vigour and passion. 

He urged states to show more "solidarity", a vexed word today, where Germans, say, complain Poles are not taking in refugees or Greeks bemoan a lack of debt relief from Germany.

And the first non-European pope in over 1,000 years reminded them of the diminishing share of the world's wealth and people in Europe. They were a "peninsula of Asia", Francis told them, urging them to remain open to the rest of the world.

Security will be tight around the muted celebrations in the Campidoglio palace in central Rome, with protesters planning to gather through the day and police on high alert after the latest attack in Europe, on Wednesday in London.

Islamic State claimed responsibility for the attack, although it was unclear what links - if any - the attacker Khalid Masood, who killed four people, had with the militant group.

The attack on the first anniversary of suicide bombings in the EU capital Brussels underlined the common threats Europeans face and drew renewed expressions of concern and reassurance on continued security cooperation between Britain and the Union.