Nigel Farage has admitted some of the pledges made by Brexit supporters in the run-up to the EU referendum were “mildly irresponsible”.

Brexiteers who spearheaded the campaign to persuade the British people to leave the EU have been heavily scrutinised after a number of pledges appear to have been backtracked on, including a Vote Leave campaign poster that promised to spend £350 million of EU funding a week on the NHS and plans to create a points-based immigration system.

When challenged over the failed promises, the former Ukip leader and prominent Brexit campaigner told Bloomberg that some pledges were “mildly irresponsible”, adding “there were lots of promises – lots of ideas get discussed at any election”.

The admissions occurred on the same day European Commission President Jean-Claude Juncker gave his annual speech to the European Parliament - which was rife with awkward truths.

With Britain walking away and some eastern nations routinely showing open hostility, the European Union's chief painted a bleak picture of the bloc and implored the 27 remaining nations to stop bickering at a time when ever more people question its relevance.

"We should admit we have many unresolved problems," he said. "The facts are plain: The world is getting bigger. And we are getting smaller."

Mr Junker used his speech to call for a joint command headquarters for EU military missions and greater defence cooperation, reviving long-running efforts to reduce reliance on the United States.

Brexit protest: Thousands march in London Show all 12 1 /12 Brexit protest: Thousands march in London Brexit protest: Thousands march in London A woman poses with a home-made European Union flag as Remain supporters gather on Park Lane in London to show their support for the EU in the wake of Brexit PA Brexit protest: Thousands march in London Remain supporters demonstrate in Parliament Square PA Brexit protest: Thousands march in London Tens of thousands of people gathered to protest the result of the EU referendum PA Brexit protest: Thousands march in London A majority of people in the capital voted to remain in the European Union Reuters Brexit protest: Thousands march in London Protesters chanted: “What do we want to do? Stay in the EU” PA Brexit protest: Thousands march in London The march follows a similar rally in Trafalgar Square that was cancelled due to heavy rain – but which tens of thousands of people turned up to anyway Reuters Brexit protest: Thousands march in London Britain voted to leave the European Union in a referendum by 52 per cent to 48 per cent Reuters Brexit protest: Thousands march in London But support for the Leave campaign in urban areas and among young people was significantly lower Rex features Brexit protest: Thousands march in London Marchers gathered at Park Lane at 11am and marched towards Parliament Square PA Brexit protest: Thousands march in London Some protesters held up baguettes in a display of affection for our continental neighbours PA Brexit protest: Thousands march in London The disparity between different parts of the country has promoted a four million signature petition calling for a second referendum and even a renewed push for Scotland to cede from the UK PA Brexit protest: Thousands march in London The event’s organiser, King’s College graduate Kieran MacDermott, wrote: “We can prevent Brexit by refusing to accept the referendum as the final say and take our finger off the self-destruct button" Reuters

He said no single EU government had a military large enough to deal with security challenges facing Europe, from Islamic militants to a more hostile Russia and echo a plan put forward this week by France and Germany.

“We must have a European headquarters and so we should work towards a common military force,” Mr Juncker said, although officials stressed this did not amount to an EU army.

“This should be to complement NATO,” he said, also outlining proposals for a common defence fund.

Mr Farage, who is still an MEP after stepping down as Ukip leader, then launched into a scathing attack on the EU, saying a “declaration of war” had been issued on any sensible Brexit negotiations.

A weary-eyed Mr Junker was seen holding his head in his hands as Mr Farage accused the European Parliament of already appointing its “divorce lawyers” by selecting Guy Verhofstadt as its chief Brexit negotiator.

Mr Farage said: "In terms of Brexit, you're probably right to be slightly critical of the British government, who ought to get on with it.

"But the EU is getting on with it and you've already appointed your divorce lawyers. On behalf of the European Parliament, we've got Guy Verhofstadt. He's the man who is going to be negotiating Brexit.

"If you were to think of this building [the EU Parliament] as a temple, well, Mr Verhofstadt is the high priest. A fanatic. In fact, there is only really one nationalist in this room and it's you [Verhofstadt] because you want flags, anthems, armies... you are an EU nationalist.

"I frankly think that this appointment amounts to pretty much a declaration of war on any sensible negotiation process.

Earlier this week, the co-chair of the official campaign to leave the EU said the pledge regarding NHS funding was only an “example” of how such funds could be allocated.

Gisela Stuart downplayed the policy, which was printed on the side of a giant red bus during the campaign, telling the BBC Daily Politics programme earlier this week: “No, the NHS was the example of that if you’re spending that amounts of money and you don’t have control of what to do with it, I would spend it on the NHS.”

Mr Farage’s recent comments echo those he made on the morning of the EU referendum result when he dismissed the pledge as a “mistake”.

Although not affiliated with Vote Leave, Mr Farage did not speak out against the figure during the campaign, but told broadcasters hours after the votes were counted: “No I can’t [guarantee spending £350m on the NHS], and I would never have made that claim. That was one of the mistakes that I think the Leave campaign made.”

No deadline to trigger Article 50 and begin the formal process for Britain to leave the EU has been set by Theresa May's Government but the Prime Minister previously said negotiations would not be started in 2016.

Mr Farage, however, said he expects the move could be taken as soon as January.

The President of the European Commission urged on Wednesday that Brexit negotiations should begin “as soon as possible”.