Britain got its first taste Wednesday of a future outside the EU as Europe's leaders met without premier David Cameron and warned London must accept EU migrants to win access to the bloc's free trade zone.

European leaders gathering without a British representative for the first time in 40 years poured cold water on the chance of Britain gaining no-strings-attached access to the huge EU single market of 500 million people.

"Leaders made it crystal clear today that access to the single market requires acceptance of all four freedoms, including freedom of movement," EU president Donald Tusk told a news conference.

The statement was a blow to "Brexit" campaigners, who promised to restrict large-scale EU migration to Britain while assuring British companies would still be able to easily sell goods and services to the continent.

It came as the domestic tremors from the referendum shock continued, with Cameron urging the embattled head of the opposition Labour party, Jeremy Corbyn, to step down, saying "for heaven's sake man, go".

Cameron, who himself is under pressure to quickly initiate divorce proceedings by formally telling the EU Britain wants to leave, attended his last EU summit in Brussels Tuesday.

French President Francois Hollande echoed Tusk's position after Wednesday's meeting of the 27 other EU members.

"If Britain wants to have common market access, like Norway for example, then the UK will have to respect... the free movement of goods, capital, people and services," he said.

German Chancellor Angela Merkel has also warned that London cannot not "cherry-pick" the terms of the exit negotiations.

Cameron has resisted pressure to immediately activate the Article 50 mechanism to leave the EU, saying he is leaving it to his successor, who will not be named until September 9.

EU leaders say that until this notification is made, no talks can begin -- formally or informally -- on resetting Britain's ties with the EU, a process meant to last two years.

After meeting Cameron on Tuesday EU leaders expressed some understanding for his predicament, but European Commission President Jean-Claude Juncker stressed that Britain cannot "meditate for months".

Some in Brussels are concerned that giving Britain favourable divorce terms will spark a domino effect of others leaving the EU, with euroscepticism growing in many member states.

In an effort to tackle this, the leaders agreed on Wednesday they need to do more to battle what a final joint statement called "dissatisfaction with the current state of affairs."

"Europeans expect us to do better when it comes to providing security, jobs and growth, as well as hope for a better future," they said, announcing a "political reflection to give an impulse to further reforms".

Meanwhile in Britain's parliament, Cameron laid into Labour leader Corbyn, who has been fighting for his political life since the June 23 Brexit vote.

"It might be in my party's interests for him to sit there, it's not in the national interests and I would say, for heaven's sake man, go," Cameron told Corbyn.

Labour MPs voted against Corbyn in a no-confidence motion Tuesday after dozens of members of his frontbench team stepped down, with the veteran left-winger accused of not campaigning hard enough against Brexit.

Within the governing Conservatives, nominations opened Wednesday for a successor to Cameron, who threw in the towel after the referendum.

Boris Johnson, the charismatic blond-mopped former London mayor and a leading Brexiteer, was expected to throw his hat in the ring along with Interior Minister Theresa May.

A new poll Tuesday put May in the lead with 31 percent, against 24 percent for Johnson.

One notable British representative was however in Brussels Wednesday, in the person of Scotland's First Minister Nicola Sturgeon, meeting European Parliament President Martin Schulz and Juncker.

Scots overwhelmingly backed "Remain" in last Thursday's vote, and the combative Sturgeon has said she was "utterly determined to preserve Scotland's relationship and place within the EU".

That may require a new referendum on Scottish independence, with Sturgeon saying that the Britain Scots voted to stay with in 2014 "does not exist any more".

However, even assuming that such a referendum is held -- which Cameron opposes -- and returns a result in favour of breaking the centuries-old union, acquiring EU membership is far from guaranteed.

US Secretary of State John Kerry meanwhile said on Tuesday that the "very complicated divorce" that is Brexit might never happen.

Asked if he thought the decision to leave the EU could be "walked back" and if so how, Kerry said, without elaborating: "I think there are a number of ways."