Sadiq Khan is to instruct the body tasked with planning for terrorism attacks and disasters such as the Grenfell Tower fire to start making preparations for a no-deal Brexit, to assess whether London could face potential shortages of medicines and food.

The London mayor said the government was dragging its feet and leaving businesses and EU citizens in limbo by refusing to guarantee their rights in the event of a no-deal Brexit.

Khan said he would ask the London resilience forum to assess the impact of no deal on the capital’s access to medicines, energy, food and emergency services, as well as on law and order. The forum is made up of 170 organisations including the emergency services, local authorities and the NHS.

The mayor said the government was being “held hostage by the hard-Brexit zealots in parliament” that made a no-deal Brexit more likely.

“If the government had taken a different approach to the negotiations this would never have been an option, but we are now left with no choice but to plan for a no-deal scenario,” he said.

The foreign secretary, Jeremy Hunt, underlined the risks of no deal on Thursday. Speaking to ITV News in The Hague, on the latest leg of his tour of EU countries, he said the implications of no deal would be “profound”.

“We have to be prepared for that; we don’t want that to happen, though. And it would be a mistake that we would regret for generations if we had a fissure – a messy, ugly divorce,” he said.

The remarks are very likely to infuriate hardline Tory Brexiters, who have insisted no deal would be manageable. Hunt also struck an upbeat tone about the likelihood of achieving a deal on the basis of the government’s Chequers proposals, saying he was “getting a very strong sense that people want to engage seriously”.

Khan’s intervention comes as Whitehall civil servants prepare to publish a slew of technical notes in the coming days on the no-deal preparations being made by a number of departments.

Papers slated for publication next week include financial services and life sciences. In some areas, such as ensuring the continuity of subsidy payments to farmers, the government is confident that it could secure continuity even in the event of no deal. But in others, such as the impact of the loss of EU “passporting rights” for British banks, the immediate impact is uncertain.

Khan said he would launch a consultation for London businesses to determine areas where City Hall could help them prepare for Brexit eventualities, but added that it was already clear the biggest threat was to the rights of European employees, who have not had their status guaranteed in the event of no deal.

Any change in their status would be hugely disruptive, Khan said. European Economic Area nationals account for about 750,000 jobs in London’s economy, including 32% of jobs in the construction sector and 11% of health and social care jobs.

Campaign for second Brexit vote seeks support beyond capital Read more

“The government’s current offer of settled status relies on a successful outcome of the Brexit negotiations,” Khan said. “If talks were to break down, the three million EU citizens living in the UK, around 1 million of whom are Londoners, would have no guarantee that their rights to stay and work would be protected.

“Businesses regularly report the difficulty they face in getting the talented workers they need, and to now threaten the Europeans they currently employ is completely and utterly reckless.”

Theresa May has previously pledged that “EU citizens living lawfully in the UK today will be able to stay”, whatever the outcome of the negotiations, and government sources said their right to stay would not be in doubt in the event of no deal, although some reciprocal rights such as access to benefits would need to be negotiated if the December joint agreement with the EU collapsed.