France, Germany, Italy and Spain have backed the idea of a multi-speed EU, as the 28-nation bloc prepares to mark 60 years since its founding treaty.
French President Francois Hollande said “Unity does not mean uniformity,” as he hosted the other leaders in the ornate Versailles Palace near Paris.
The EU Commission accepts that projects do not have to involve all EU members.
Brexit – a psychological and budgetary blow – now overshadows the anniversary, the BBC’s Kevin Connolly reports.
EU leaders are focusing on a strategy of promising both deeper co-operation but also the possibility of different member states joining common projects at times that suit them.
In some respects the EU is already a multi-speed organisation. Not all members are in the Schengen border-free zone, and 19 of the 28 use the euro.
The UK, poised to trigger the withdrawal mechanism soon, already has many opt-outs from EU policies.