THERESA May is “clueless” over the UK’s negotiating stance and should include Nicola Sturgeon in the talks, a member of the European Parliament’s Brexit team has said.

Philippe Lamberts also warned that Britain would be more vulnerable to terror attacks and “irrelevant” on the global stage if it crashed out of the EU without deals on security and trade.

The Belgian MEP, a member of the parliament’s Brexit Steering Group, urged the UK to hold a referendum on any exit terms agreed with the bloc to allow its citizens a final say.

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”Asked if the First Minister should be involved in the talks, Lamberts said: “When you negotiate something as momentous as a potential exit from the European Union and when you know that there was quite a large degree of disunity in the United Kingdom, it makes total political and democratic sense to involve all parties concerned.

Theresa May has so far not proven to have been very inclusive in the way of managing [the process]. So far she has appeared to be a bit clueless to me. She has appeared not to be able to seize the moment and make sure she has broad support so she can negotiate her position.”

He added: “The dangers of the derailment of the process are numerous and it’s in no-one’s interests, certainly not the UK’s interests, to fall off the cliff in March 2019.”

Lamberts spoke out after the third round of exit talks in Brussels where the European Union’s chief neg- otiator, Michel Barnier, delivered a scathing verdict on the UK’s stance on the financial settlement, citizens’ rights and hopes for future access to the single market, describing them as “nostalgic and unrealistic”.

The steering group, chaired by another Belgian MEP, Guy Verhofstadt, is a small, highly influential team which liaises closely with Barnier. It is responsible for preparing the negotiations on behalf of the European Parliament and meets with Barnier ahead of every round of negotiations.

During his exclusive interview with The National, Lamberts also raised the issue of British vulnerability to terror attacks post-Brexit and mocked the Prime Minister’s notion that Britain would be a major player on the world stage.

“The UK will be a tiny global player – irrelevant,” he said.

“Unless you have a very strong agreement with the European Union, in terms of managing security, then of course the UK will be on its own.

“And look at what happens when EU member states are not co- operating. The terrorists manage to find a hole in our mettle, whereas when we co-operate we are much more efficient.

“So again, it’s not just a matter of safety, it’s a matter of influence. The Brexiteers are deluding themselves if they believe Great Britain on its own will walk proud.”

Lambert said he believed people in the UK were beginning to realise the scale of the negative impact of exiting the EU, and believed the best course of action for Britain would be to hold a referendum on any exit deal.

“I think it is gradually dawning on people what the ramifications of exit are,” he said.

“I think the negotiations will show that one, Brexit is very hard to manage, and second, that it is a negative for the United Kingdom.

“And whatever deal we might come up with will, I believe, be worse than what the UK has now.

“The right thing to do in my view would be to negotiate an exit deal and submit the result of the negotiations to a referendum. I believe that, democratically, that would be the proper way to do it.”

A Scottish Government spokesman said: “The people of Scotland voted overwhelmingly to remain within the European Union. It is only fair, therefore, that at the end of the Brexit process, people here should have a choice about our future direction as a country.

“These comments add to the growing support for Scotland and the other devolved governments having a place at the negotiating table.

“Scotland is an outward-looking country and we share the values of solidarity, co-operation and democracy which underpin the EU.”

A spokesperson for the UK’s Department for Exiting the European Union said: “The British people voted to leave the EU in last year’s referendum and we are delivering on that decision.

“We are committed to securing a deal that works for the entire United Kingdom – for Scotland, Wales, Northern Ireland and all parts of England. We will continue to engage the devolved administrations as we seek to secure that deal.

“The Scottish Government has been clear that continuing to work with the EU to preserve European security and to fight terrorism is of the utmost importance.”