Phil Hogan, who is expected to be confirmed as the EU's new Trade Commissioner later, has said Boris Johnson now has 10 days to come up with credible ideas for a new Brexit deal.
It is reported there are plans by the British Government to create customs posts along both sides of the border to replace the backstop.
It would lead to the posts being built between five and 10 miles back from the current border.
Mr Hogan said many EU officials are now wondering when the next extension to the deadline will be.
He said: "If you don't get a deal, then what will you pass legislation for to have an extension? Provided Mr Johnson agrees to the law of the land.
"We're talking about 10 days to try and do a deal."
Mr Hogan said he does not agree that a proposed trade deal between the EU and South American countries would be devastating for farmers.
The agreement has been strongly criticised by the beef sector, which is worried about sub-standard product being imported from Mercosur countries.
The issue was high on the agenda at the nomination hearing for Phil Hogan as the EU's trade commissioner last night.
He believes the deal can give the EU more power to stand up to Brazil on the destruction of the Amazon rainforest:
Phil Hogan said: "All of us are appalled by what we see in the Amazon, but we don't have the tools at the moment in the European Union to deal with him effectively without actually some leverage to a trade policy.
"I think that the final ratification stage is where you actually can have a lot of leverage."