Media playback is unsupported on your device Media caption Gove expresses doubts about Brexit customs plan

Michael Gove has cast doubt on the viability of Theresa May's proposed customs partnership with the EU after Brexit, telling the BBC it has "flaws".

The environment secretary said there were "significant question marks" about whether the model, described as crazy by Boris Johnson, was deliverable.

The PM, whose ministers are considering two customs options, says "compromises" will be needed but she will "deliver".

But Labour said the lack of a decision on the critical issue was "farcical".

The UK is committed to leaving the current customs union when it exits the EU on 29 March 2019 and ministers are under pressure to agree soon on a successor arrangement amid divisions in cabinet.

The customs partnership thought to be preferred by Mrs May would see Britain continue to collect tariffs on behalf of the EU, as well as refunding UK firms if they were only liable for lower rates.

'Trust me'

Writing in the Sunday Times, Mrs May promised a solution which ensured frictionless trade, enabled the UK to strike trade deals around the world and which did not result in a hard border on the island of Ireland.

"You can trust me to deliver," she wrote. "I will not let you down".

On the customs issues, she said she had "proposed different options for a new customs arrangement with the EU and the government will continue to develop them during the negotiations".

What are the government's options?

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A 'highly streamlined' customs arrangement - This would minimise customs checks rather than getting rid of them altogether, using new technologies and things like trusted trader schemes, which could allow companies to pay duties in bulk every few months rather than every time their goods crossed a border

A customs partnership - This would remove the need for new customs checks at the border. The UK would collect tariffs set by the EU customs union on goods coming into the UK. If those goods didn't leave the UK and UK tariffs on them were lower, companies could then claim back the difference.

Mr Gove, one of the leading Brexiteers in the cabinet, told the Andrew Marr show that he trusted the PM to reach an agreement but he expressed doubts about what is believed to be her preferred option.

The leading Leave campaigner is one of a group of cabinet ministers asked by the PM to stress-test the partnership model - which would see the UK collect tariffs set by the EU on goods coming into the UK.

'Novel'

"Because it is novel, because no model like this exists, there have to be significant question marks over the deliverability of it on time," he said.

"More than that, what it requires the British government to do is, in effect, act as the tax collector... for the European Union... It is my view that the new customs partnership has flaws and that they need to be tested."

Image caption The Irish foreign minister said he "did not take his lead" from Boris Johnson

Neither this option nor the alternative - a "maximum facilitation" arrangement using technology to ensure the seamless movement of goods - was "absolutely perfect", he acknowledged.

But Labour's Sir Keir Starmer said neither of the options were "workable nor acceptable to the EU" and, with time running out, Parliament must force the government's hand by backing a "comprehensive" customs union.

"We are in a farcical situation," he told the Andrew Marr show. "We want a vote on a customs union with the EU. That is the only way to resolve this impasse."

Mrs May has insisted the final Brexit deal must honour the agreements in the Northern Ireland peace process - and that there can be no hard border between Northern Ireland and the rest of the UK.

'Shared space'

But Simon Coveney, the Irish foreign minister, dismissed the idea of using any form of infrastructure or technology to maintain separate customs regimes between the Republic of Ireland and Northern Ireland.

He told the BBC's Andrew Marr Show: "We just simply think it won't work. If you don't believe me on it, listen to people who are living locally.

"Listen to the chief constable of the PSNI. He is saying any infrastructure on the border, any physical infrastructure on the border, is going to represent a risk to his officers."

Suggesting the Irish government "did not take its lead" from Boris Johnson and warning of a "difficult summer", he said the two governments could agree a "shared customs space or shared customs territory".

In other Brexit developments, a cross-party group of former cabinet ministers - including former Liberal Democrat leader Nick Clegg and former Labour foreign secretary David Miliband - has urged Parliament to vote to remain in both the customs union and the single market, arguing it is vital for the economy.

And student organisations representing almost a million young people have also written to MPs demanding a referendum on any final Brexit deal.

Organised by the campaign group For our Future's Sake, the letter from 60 student unions says promises made by pro-Brexit groups have not been kept and that an estimated 1.4 million people were too young to vote in June 2016 but now deserve a say.