Britain would be “completely killed” in global trade negotiations if it voted to leave the EU because it would be reduced to the status of Jersey and Guernsey, the French economy minister, Emmanuel Macron, has said.

As David Davis said David Cameron should be stripped of his right to negotiate a British exit in the event of a vote to leave, Macron warned that major powers such as China would have little interest in negotiating with the UK outside the EU.

Macron, appearing on BBC1’s Andrew Marr Show to highlight the new political grouping he has established in France, raised Tata’s decision to abandon its UK steel operations to illustrate the dangers of a British exit. This includes selling off its Port Talbot steel plant, the UK’s largest, which employs 4,000 workers.

Macron said Britain would have an even weaker hand in its negotiations with China, which is blamed for the crisis in the British steel industry after dumping cheap produce in Europe, if it were not part of a 500 million-strong trading block.

Macron said: “Your decision to be taken is much more about your role in globalisation. I think UK is not about becoming Jersey or Guernsey. Today, you are strong because you are part of the EU. When you discuss your steel industry with China you are credible because you are part of the EU, not because you are just UK. You will be completely killed otherwise.

“You will never be in the situation to negotiate face to face with the Chinese because your domestic market is not relevant for the Chinese in comparison with their domestic market. EU is the first global domestic market.”

Macron’s argument highlights a key argument of the remain campaign. This is that Britain is able to secure stronger trading deals as a member of the EU, with a population of 500 million, rather than as a single nation of 63 million. Trade policy is negotiated at an EU level.

He spoke out as David Davis said Cameron could remain as prime minister after a vote to leave the EU but would have to hand the breakup negotiations with Brussels to a minister who had supported a British exit. In a sign of how the prime minister’s personal future could become an issue during the referendum campaign, Davis took issue with the former chancellor Kenneth Clarke, who said Cameron would be out of No 10 within 30 seconds if Britain left the EU.

David Davis: ‘If we vote for Brexit then David Cameron can’t lead the renegotiations.’ Photograph: AFP/Getty Images

Davis said: “If we vote for Brexit then it is clear that David Cameron can’t lead that bit of his government’s activities, the renegotiations. He would have to appoint somebody who the public had faith in, his party had faith in but, most importantly, who believed in the negotiation. If he did that I don’t see any reason why he shouldn’t go on.”

Asked whether he, a supporter of a British exit, should lead the negotiations, David said: “Oh, go away. Let’s confine ourselves to the plausible.”

Downing Street is hoping that the campaign to keep Britain in the EU will receive a boost this week when Barack Obama endorses Britain’s membership of the EU. Boris Johnson has accused the president of hypocrisy because the US would never agree to surrender sovereignty in the way EU member states do.

Chris Grayling, the leader of the House of Commons, who supports a British exit, said Obama did not understand the nature of the pooling of sovereignty among EU member states. He said: “I don’t for a moment think that President Obama would tolerate a situation where the US gave away as much of its sovereignty as we have to Brussels. It is inconceivable. In my view, he perhaps doesn’t understand the nature of the transfer of power that has taken place. I can only think he doesn’t realise.”