Senior Tory backbencher Bernard Jenkin said the plan would mean the UK acting as a “tax collector” for the EU and would fail the Prime Minister’s key tests for a deal with Brussels.
His warning came amid renewed tensions among senior ministers as members of Mrs May’s Brexit “war cabinet” prepare for a crunch meeting on Wednesday to settle their position on the UK’s future customs relations with EU.
The customs partnership will be one option on the table along with the so-called “max fac” - or maximum facilitation - solution, using technology to minimise the need for border checks.
Ahead of the meeting, the Prime Minister is reported to have been warned she could be ousted unless she sacks her chief Civil Service Brexit negotiator Oliver Robbins, seen as the leading exponent of the partnership option.
The Sunday Times quoted a “close friend” of Brexit Secretary David Davis as saying he had compared Mr Robbins to Margaret Thatcher’s controversial economic adviser Sir Alan Walters, who alienated the cabinet setting in train the events which led to her final downfall.
Government Chief Whip Julian Smith appeared to defend Mr Robbins, tweeting it was “deeply unfair” to criticise individual officials, and that the Civil Service was providing “world class” advice and support on Brexit.
His comments were subsequently re-tweeted by Mrs May’s chief of staff Gavin Barwell in what will be seen as a show of support by No 10.
Proponents of the customs partnership - which would see the UK continuing to collect tariffs on behalf of the EU for goods coming into the country - is the best way to avoid the need for a return to border checks between Northern Ireland and the Republic.
In contrast, the “max fac” solution - dubbed the “fantasy island unicorn model” by critics - has been dismissed by the EU side as completely unworkable.
However Mr Jenkin made clear the partnership option was a non-starter as far as Brexiteers were concerned.
“It doesn’t pass any of the key tests that the Prime Minister has outlined in her statements,” he told BBC1’s Sunday Politics programme.
“It doesn’t give us full regulatory autonomy so we wouldn’t be able to do free trade deals with other countries.
“Even if we did a free trade deal with another country it would be very unattractive because we would be asking them to eliminate their tariffs but we would still be charging the EU tariff when the goods came into this country.
“We would still be substantial net contributors to the EU budget which the Prime Minister has said should end. We would still be the tax collector for the European Union and that’s not acceptable.”
The partnership model is also reportedly opposed by the leading Cabinet Brexiteers including Boris Johnson, Liam Fox and Michael Gove, who is said to have described it as “completely bonkers”.
In contrast, Conservative Party chairman Brandon Lewis suggested it could provide a workable solution.
“I think it is important that we are able to get opportunities of trade deals around the world, good trade deals with our partners and friends in Europe, and that is what a good partnership will deliver and I believe that is what the Prime Minister and David Davis will deliver for this country,” he told BBC1’s The Andrew Marr Show.