Sir Keir Starmer has delivered his strongest rejection yet of calls for Britain to return to the EU customs union, warning that doing so would “unravel” recent trade pacts with the US on cars and pharmaceuticals.

Starmer’s comments suggest he favours Britain’s post-Brexit freedom to strike deals around the world — notably with President Donald Trump — over the complexity of negotiating a return to a European customs area.

The prime minister also wants to focus on existing talks with Brussels to remove barriers to trade in areas such as food and energy — along with a youth mobility scheme — rather than start a wider unpicking of Brexit.

On Wednesday Nick Thomas-Symonds, Starmer’s European relations minister, travelled to Brussels for what he described as “productive” talks on the new arrangements.

Starmer, speaking in the House of Commons, said he wanted “a closer relationship than the one we have at the moment” but warned that a return to a customs union would undo recent deals with Trump, which were particularly beneficial to carmaker Jaguar Land Rover.

“Having now done significant trade deals with other countries, including the US and India, which are hugely important to the JLR workforce and on pharma, it is not now sensible to unravel what is effectively the best deal with the US that any country has got,” Starmer told MPs.

Sir Ed Davey, Liberal Democrat leader, said: “The truth is that this government will not succeed unless they get our economy growing strongly again, and the best way to do that is a customs union with Europe.”

Re-entering a customs union with the EU, a move supported by some Labour MPs, would require the UK to at least match the bloc’s tariff rates, according to Joël Reland of the UK in a Changing Europe think-tank.

While this could deliver some economic benefits, it would have knock-on effects for the UK’s post-Brexit trade deals, which have offered lower tariffs to some countries, including Australia, India and the US, he said.

“A customs union would require the UK government to give up its ‘independent trade policy’ — unwinding any concessionary tariffs which undercut EU rates,” Reland said.

“If the EU were to slap major tariffs on US goods, the UK would have to do likewise. The era of using lower tariff rates to draw concessions from the White House would be over,” he added.

Thomas-Symonds’ meeting with his EU opposite number Maroš Šefčovič aimed to keep “reset” talks on track after the collapse of discussions last month on the fees for Britain to join a €140bn EU defence fund.

Despite the warm words about “productive” discussions, significant gaps remain on the issue of a youth mobility scheme to give 18-30-year olds from the UK and the EU the opportunity to live, study and work in each other’s countries for a set period.

The UK continues insist that any scheme must be “capped”, while resisting Brussels’ demands for ‘home fees’ for EU students at British universities.

A draft text of the EU’s proposal for a “youth experience scheme”, seen by the Financial Times this month, put the two sides publicly at odds over the issue, threatening progress in other areas, such as a so-called veterinary agreement to remove border checks and relinking EU and UK carbon pricing schemes.