Cabinet tensions over Brexit erupted again 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 the Cabinet split over which of two customs models to back, environment secretary Mr Gove said 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".
Mr 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."
Mr Gove added: "It's my view that the new customs partnership has flaws and they need to be tested."
The Environment Secretary said he was against any extension of the current customs union in order to give more time to find a new system.
Mrs May has set up two Cabinet groups to consider the customs options.
The customs arrangement with the EU that Mr Johnson opposes would see the UK collecting 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 get trade to flow smoothly with the EU after Brexit.
Writing in The Sunday Times after weeks of Cabinet wrangling, Mrs May said: "You can trust me to deliver. I will not let you down."
Mrs May stressed the UK would be aligned with Brussels on some issues as there had to be "compromises" after withdrawal.
"Of course, the details are incredibly complex, and, as in any negotiation, there will have to be compromises."
Tory former leader Iain Duncan Smith warned party MPs not to vote to stay in the customs union with the EU as some 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."
He added: "What we propose is a combination, on the one hand a comprehensive customs union, nobody credible suggests you can achieve no hard border [in Ireland] without it and also a strong single market relationship that hard wires the benefits of the single market into the future agreement."
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 on Sunday, 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."
