Michel Barnier slapped down Sir Keir Starmer’s plan to rewrite the Brexit deal.
The bloc’s former chief negotiator warned there is “no room for any kind of unravelling of the single market”.
He suggested there could only be minor tweaks to the trade agreement, which is up for review in 2025.
The Labour leader has vowed to strike a “much better” deal with Brussels if he wins the next general election.
But Mr Barnier told ITV’s Peston show: “To be frank there is no room for any kind of renegotiation.”
The former EU heavyweight added: “As I always said during all the negotiations from day one to the last day, there is no room for any kind of unravelling of the single market.
“So that means if his point is very clear we can improve technical detail, for instance veterinary relations, but we have to be out or in.”
Mr Barnier frequently warned the UK cannot “cherry pick” benefits during tortuous Brexit negotiations following the 2016 referendum.
Sir Keir, a Remainer who backed a second referendum, earlier this year pledged to seek a “much better” agreement with the EU and criticised the current pact as “far too thin”.
But he has claimed he would not look to rejoin the single market or customs union.
Speaking in September, Sir Keir said: “Almost everyone recognises the deal Johnson struck is not a good deal – it’s far too thin. As we go into 2025 we will attempt to get a much better deal for the UK.”