Ten years on, Brexit disputes still hold back Britain's reconciliation with EU
Charles Grant, director of the Centre for European Reform (CER) think-tank in London, said progress had been "slow and painful", reflecting the broader mistrust that still exists.
The CER's Grant said officials in Brussels were currently hostile to the idea, although that could change over time if geopolitical challenges mount.
But Grant said Brussels would be hamstrung by Marine Le Pen's National Rally in France and Germany's AfD who could demand their own opt outs if they ever got into power. At the same time, in Britain the veteran Brexit campaigner Nigel Farage has led opinion polls for more than a year and has threatened to tear up any closer agreement.