The president of the European Parliament has warned the UK there will be "no a la carte menu" when it comes to reaching a Brexit deal with Brussels.
Martin Schulz stressed the best possible agreement with the European Union was membership of the bloc and that any other arrangement "necessarily entails trade-offs".
In a slapdown to claims by British foreign secretary Boris Johnson that the UK leaving the EU did not need to take two years, Mr Schulz said that "many tend to underestimate the complexity and delay" in forging new trading links.
Mr Schulz also underlined the key role that the European Parliament would have in the renegotiated agreement between the EU and UK, warning it could veto any final deal.
In his speech at the London School of Economics in London, he reiterated that Article 50 - the official mechanism for beginning the process of leaving the EU - was "not a mere formality".
He said the longer the UK delayed "the more entrenched the respective positions risk becoming".
Mr Schulz comments came during a visit to the UK to discuss Britain's departure from the EU.
As well as meeting prime minister Theresa May, he had also held talks with Labour leader Jeremy Corbyn.
In his address to students, Mr Schulz referred to the Brexit vote as an "unfortunate hand of cards".
He said the best possible deal with the EU is membership of the EU.
"Any other arrangement necessarily entails trade-offs.
"Perhaps it is easier to convey what I am trying to say in culinary terms - there will be no a la carte menu."
In an explicit threat to block any agreement, Mr Schulz said: "The institution which I have the honour of presiding since 2012, the European Parliament, will play a key role in setting the new relationship; between the EU and the UK - not least because we must consent to any withdrawal treaty and subsequent treaty setting out the full relationship."
He highlighted previous arrangements "where the Parliament's priorities were not reflected in the negotiations and it decided to veto the final deal".
Mr Schulz also restated the principle of "no negotiation before notification".
"We first need the UK to tell its partners formally what future it sees with them," he said.
