Michel Barnier has stressed there can be "no unravelling" of the single market, as UK ministers try to agree on a future UK-EU relationship.
The EU's chief negotiator said "realistic and workable" UK proposals are needed, as the cabinet meets at Chequers, for what happens post-Brexit.
Theresa May is expected to propose keeping the UK aligned with the EU on trading rules for goods, not services.
If the cabinet agree, proposals will then go to the EU.
But Mr Barnier warned in a speech: "The single market is our main economic public good. There will be no damaging it, no unravelling what we have achieved together with the UK."
He also repeated warnings that time is running out to reach a deal.
"Time is short. We need to quickly have realistic and workable solutions and obviously we look forward to the UK's White Paper.
"The UK's proposals will facilitate both the UK's internal political debate and negotiation with us.
"Our objective has always been to find an agreement with the UK, not against the UK."
Talks with the EU continue over the terms of the UK's withdrawal from the EU in March next year.
What happens to the Irish border remains a sticking point.
Both sides want a deal to be reached by the next European Council meeting in October.
Mr Barnier also said there was a need to "de-dramatise" the issue of the "backstop" - the EU's proposal to avoid a hard border between Northern Ireland and the Republic of Ireland, if the EU and UK cannot agree new arrangements in time.
Under the EU's proposals "a limited set of EU rules would continue to apply in Northern Ireland as they do today", Mr Barnier said in a speech.