UK Cabinet tensions over Brexit erupted again yesterday as Michael Gove said there were “significant question marks” over the customs partnership option favoured by the Prime Minister.

The remarks came after a plea by Theresa May for unity as she insisted she could be trusted to deliver the Brexit people voted for.

With ministers split over which of two customs models – neither of which has found favour with the EU – to back, the Environment Secretary said that neither option being considered was perfect.

Appearing on BBC One’s The Andrew Marr Show, he was pressed on whether Foreign Secretary Boris Johnson was right to brand the customs partnership option as “crazy”.

Gove said: “Across Government, across Cabinet, there is agreement that neither of these two models is absolutely perfect. And with the new customs partnership, Boris pointed out that because it’s novel, because no model like this exists, there have to be significant question marks over the deliverability of it on time.

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“It’s my view that the new customs partnership has flaws and they need to be tested.”

Gove added that he was against any extension of the current customs union in order to give more time to find a new system.

May has set up two Cabinet groups to consider the customs options.

The customs partnership with the EU that May favours and Johnson opposes would see the UK collecting EU tariffs on behalf of Brussels.

An alternative option called maximum facilitation, known as “max fac”, would rely on new technology and trusted trader schemes to try to get trade to flow smoothly with the EU after Brexit.

Writing in a Sunday newspaper after weeks of Cabinet wrangling, May said: “You can trust me to deliver. I will not let you down.”

She stressed that the UK would be aligned with Brussels on some issues as there had to be “compromises”.

“Of course, the details are incredibly complex, and, as in any negotiation, there will have to be compromises,” she added.

Former Tory leader Iain Duncan Smith, a prominent Eurosceptic, called for his party’s MPs not to vote to stay in the customs union with the EU, as some Remain-supporting rebels have urged, telling the BBC: “It was in the manifesto and all my colleagues stood on that.

“So this is a very big issue if they’re deciding to break this.

“Because they do literally plunge a knife into the heart of Government and particularly to the Prime Minister – because it is very much her fixed view, and that is what we stood on at the last election.”

Shadow Brexit secretary Sir Keir Starmer branded Cabinet divisions over proposals for a customs arrangement farcical.

He told the BBC: “I think we are in a farcical situation at the moment, nearly two years after the referendum the Cabinet is fighting over [the] two customs options, neither of which frankly are workable, neither of which are acceptable to the EU.”

Meanwhile, leading nurses have said the right to freely cross the border has to be maintained to protect essential healthcare services.

The Royal College of Nursing (RCN) said an estimated 30,000 workers cross the Irish border each day and must be able to travel freely to care for patients.

In a keynote address to the RCN annual conference in Belfast yesterday, chief executive Janet Davies said: “No matter where they live – no matter where they work – our nurses must be able to continue to travel freely in order to care for patients.

“It is of paramount importance that this right remains protected here across the island of Ireland.”

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It has also emerged that Scottish universities could lose out on millions of pounds worth of funding after Brexit.

The SNP’s Joan McAlpine, who convenes Holyrood’s Culture and Europe Committee, said UK research bodies could lose out on access to the European Commission’s research budget. Funding for Horizon for Europe is set to rise by almost 30 per cent after 2020.

“Scottish and UK researchers are set to receive over €11 billion by the end of the current Horizon programme [in 2020] – but unfortunately we will lose out next time when the research budget is likely to increase by almost 30 per cent,” said McAlpine. “The UK Government needs to recognise the folly of its extreme Brexit plans, and the damage they are doing across important sectors of our economy, before it is too late.”