Mme Loiseau told Public Sénat television: “There is no way we will wait [for the UK] – wait because [Prime Minister] Boris Johnson has found yet another excuse to not do what he will eventually end up doing.
“We have seen since the very beginning that Mr Johnson has a tendency to say one thing and do another.”
Mme Loiseau added the Johnson government would have no choice but to “nominate a commissioner”.
She continued: “I really hope we will not have to wait [for London].”
The bloc launched a legal case against Britain last week after London, in the midst of a messy Brexit-focused election campaign, said it would not name a representative for the new Commission.
The bloc’s executive arm said in a statement: “The European Commission has today sent a letter of formal notice to the United Kingdom for breaching its EU treaty obligations by not suggesting a candidate for the post of EU Commissioner.”
It has given London until November 22 to respond.
The UK rejected calls to name a candidate for a British commissioner earlier this month, arguing that it could not make international appointments in the run-up to its crunch general election on December 12.
A British government spokesman said: “We have written to the EU to confirm that pre-election guidance states the UK should not normally make nominations for international appointments during this period.
Brussels, for its part, insists that Britain has to nominate a commissioner because it is still a full member of the EU.
London failed to propose a candidate for when the other 27 EU countries did because Mr Johnson was adamant the UK would no longer be a member of the bloc in November.
But he was forced by his own parliament to ask for a delay to Brexit until January 31, throwing back open the question of Britain proposing a member of the EU executive.
Some EU officials believe that the UK’s refusal to put forward a candidate even though it has not yet left the bloc might put the new Commission in legal difficulty as EU law stipulates that there should be one commissioner per member state.
The bloc, however, could circumvent the problem with a decision by the EU27 not to apply current rules, though risks of legal challenges remain.
The new executive had been scheduled to start on November 1, but the rejection of France, Hungary and Romania’s candidates and the absence of a British nominee made that impossible.
But the European Parliament is expected to endorse the entire Commission in a next week after the three countries’ new picks were approved by MEPs on Monday.
The Commission, whose powers include introducing international trade deals, policing member states’ finances and proposing EU-wide laws on topics ranging from the environment to migration, is due to relaunch on December 1 under the new leadership of German conservative Ursula von der Leyen.
London’s commissioner snub, however, has underlined how Brexit has rattled the EU and how the bloc’s increasingly divided political landscape has complicated the building of consensus and compromise crucial to making decisions and advancing policy.