European Parliament President Martin Schulz took a swipe at Boris Johnson as he addressed a group of lecturers and students in London this afternoon (23 September), reigniting the war of words between the EU and the UK's foreign secretary.

Schulz said it was "difficult to digest" what Vote Leave campaigner Johnson said ahead of the historic ballot, while suggesting Theresa May's attitude towards the bombastic Conservative and his negotiating skills were revealed with her appointed of David Davis as Brexit secretary.

"It was, time to time, difficult to digest what he said during the campaign, but I'm full of respect for the foreign minister of the UK," Schulz said.

The EU chief was reacting to Johnson's claim yesterday that it was "complete baloney" that the UK would have choose between tougher immigration rules and single-market access.

"The two things have nothing to do with each other. We should go for a jumbo free trade deal and take back control of our immigration policy," he told Sky News.

But the EU presidents, including Schulz, Jean-Claude Juncker and Donald Tusk, have continually said Britain cannot be given "single market a la carte" as part of its deal with Brussels.

Schulz, who has met with May, Mayor of London Sadiq Khan and Labour leader Jeremy Corbyn during his visit, also said he urged the prime minister to trigger Article 50 of the Lisbon Treaty – the official mechanism to split from the EU – "as soon as possible".

"It is understandable that such a complex task as the triggering of Article 50 is not a mere formality and requires detailed preparations which could start after the referendum result," he told the London Schools of Economics audience.

"However, it is also conceivable that the longer the UK waits to take this step, the more entrenched the respective positions risk becoming. That is why I called on Prime Minister May to notify the UK's departure from the EU as soon as possible."

Schulz warned that the EU could not "press the pause button" on its post-referendum activities, with numerous elections approaching in the 27 other nations and European Parliamentary votes scheduled for 2019.

"If Article 50 is triggered too late, we run the risk of facing European elections in the UK in 2019 at the very same time as it is leaving the EU. That would be a very difficult thing to explain to UK and to European citizens," he said.