It will be called a "delegation" - not an embassy - and will be part of the EU's foreign policy arm, the European External Action Service (EEAS).
There will also be a mission with five staff in Belfast to oversee the implementation of the withdrawal agreement in Northern Ireland - if there is a Brexit deal.
The plans will be discussed next week.
They will be presented for approval by ambassadors from the 27 remaining EU countries on Wednesday.
The European Commission has offices in all member states. Currently it has a team of 27 staff based at Europe House, in Smith Square, Westminster.
Once the UK leaves the EU in March next year, that office will be replaced by a new London outpost for the EEAS.
It will be about a third of the size of its equivalent in Washington DC, which has 90 personnel - although only 30 of them are classed as European diplomats.
Whether the delegation will stay in the Smith Square building - the former headquarters of the Conservative Party - is not yet known.
However, the European Commission's offices in Scotland and Wales are likely to close.
The new European Union ambassador to the UK will be appointed at a later date by its foreign policy chief Federica Mogherini.
Being Brussels' man or woman in London could be a plum job, or the occupant could find themselves sidelined in future negotiations with London.
Earlier this year, the European Commission denied that Martin Selmayr - its secretary-general and a former aide to its president Jean-Claude Juncker - was a potential candidate.
The EU has 140 delegations to countries and organisations, including the United Nations and the World Trade Organization.
Its External Action Service was established by the Lisbon Treaty to develop and deliver the EU's common foreign and security policy.