The European Union has warned London not to overdo its campaign to independently negotiate trade deals around the world while still being a member of the bloc, according to a letter to the British ambassador to Brussels seen by Reuters.

The promise of a "global Britain" assertively pursuing its interests in bilateral trade agreements around the world was central to the Brexit campaign. But, three years after Britons voted to leave, the fate of Brexit hangs in balance.

As long as it remains an EU member, Britain is at liberty only to discuss — not enact — future trade deals, and must keep the bloc informed.