Michel Barnier told reporters today that a long extension to Article 50 would only be considered if there was “a new political process” in the UK to warrant it.
He also cautioned that the EU will need to know “the reason and the usefulness” of any UK request for a delay to Brexit before deciding whether to grant an extension.
And he said EU leaders would need a “concrete plan from the UK in order to be able to make an informed decision.”
It came as the Prime Minister, who earlier admitted the country was in “crisis”, was due to write by the end of Wednesday to European Council president Donald Tusk, setting out her proposal to extend the two-year process of negotiating withdrawal under the EU’s Article 50 rules.
But a delay to Brexit beyond the scheduled date of March 29 needs the approval of all 27 remaining member states at a summit in Brussels on Thursday.
Mr Barnier told a news conference in Brussels: “It is our duty to ask whether this extension would be useful because an extension will be something which would extend uncertainty and uncertainty costs.”
Unconfirmed reports suggest that Mrs May could ask for a lengthy extension to Article 50, with the option of an early break in May or June if she manages to get her Withdrawal Agreement through Parliament.
But Mr Barnier appeared to pour cold water on this possibility, telling a reporter: “You said both short and long.
“Well, it’s either one or the other, isn’t it?”
He added: “My feeling is … a longer extension needs to be linked to something new.
“There needs to be a new event or a new political process.”
Mr Barnier said that in making the decision, EU leaders would have to assess what is in the best interests of the EU.
He continued: “Does an extension increase the chances for the ratification of the withdrawal agreement? Will the UK request an extension because it wants a bit more time to rework the political declaration (on future EU-UK ties)?
“What would be the purpose and the outcome of an extension? How can we ensure that at the end of a possible extension we are not back in the same situation as today?”
Mr Barnier said the British Government and parliament must decide quickly on how to move forward.
“The European Council will need to assess what is in the best interest of the EU,” he said.
“Extending uncertainty without a clear plan would add to the economic costs to our businesses but could also incur a political cost for the EU.”
If the EU agrees an extension, Brexit will be delayed by the passage of a statutory instrument through both Houses of Parliament removing the date of March 29 from the legislation.
MPs will be given an opportunity in the House of Commons on Monday to debate how the process should go forward.
At a meeting of Cabinet in Downing Street, Mrs May voiced her “absolute determination” that MPs should have another chance to vote on her Brexit deal, despite the bombshell intervention of the Commons Speaker.
John Bercow provoked uproar at Westminster on Monday when he ruled that the Government cannot bring the Prime Minister’s deal back for a third “meaningful vote” unless there were substantial changes.
However, in the course of a 90-minute discussion at the weekly meeting of the Cabinet in Downing Street, Mrs May made clear she wanted MPs to have another vote “as soon as possible”.
The Prime Minister’s official spokesman said: “What you can see from the Prime Minister and her colleagues is an absolute determination to find a way in which Parliament could vote for the UK to leave the European Union with a deal.
“The Prime Minister has been very clear throughout that she wants that to happen as soon as possible.”
Nevertheless, there was said to be concern among some ministers that Brexit appeared to be slipping away.
The Leader of the House Andrea Leadsom is understood to have told the meeting: “This used to be the Cabinet that would deliver Brexit and now from what I’m hearing it’s not.”
German Chancellor Angela Merkel said Brexit was in a state of “flux” and that EU leaders will “try to react” to whatever Mrs May proposes on Thursday.
The Prime Minister has previously said if her deal is not agreed there will have to be an extended delay to Brexit, with the UK staging elections to the European Parliament in May.
However, her spokesman said: “She has said in the House of Commons that she does not want there to be a long delay and that she believes asking the British public to take part in European elections three years after they voted to leave the EU would represent a failure by politicians.”
Downing Street confirmed discussions were continuing with the Democratic Unionist Party, which props up the Government at Westminster, in an effort to build support for the deal after last week’s 149-vote defeat.
And Mrs May held talks on Brexit with former foreign secretary Boris Johnson, one of the most strident critics of her Withdrawal Agreement.
Jeremy Corbyn met leaders of the other opposition parties, but declined to sign up to joint action on a Brexit alternative.
Green co-leader Caroline Lucas said the chances of a People’s Vote were “moderately higher” after the meeting, also attended by the SNP, Liberal Democrats and Plaid Cymru.
A Labour spokesman said: “Should there not be a majority in Parliament for May’s deal or a public vote, Corbyn called on the other parties to engage constructively to find a parliamentary majority for a close economic relationship with the EU that can work for the whole country.”
Meanwhile, Mr Tusk met Irish premier Leo Varadkar in Dublin to discuss the backstop arrangement which both the EU and Ireland insist is necessary to keep the border open.
In a joint statement, they said Mr Tusk had expressed “strong and ongoing solidarity” with Ireland and they had agreed they needed to see what proposals would now emerge from London.