Theresa May has been put on the defensive over Brexit at a business summit in New York after a leading chief executive asked her to explain the possible risks of Britain’s departure on future investment, saying: “How bad can things get?”

The question came from Steve Schwarzman of asset management firm Blackstone. He described Brexit as “a little daunting”, and asked May about what he said were the risks of a change of government in the UK.

The questioning came at the end of an otherwise straightforward event for the prime minister, as she sought to reassure executives with a speech in New York about the post-Brexit business landscape.

Even after leaving the EU, the UK would be “a fully engaged and ambitious partner”, with continued strong links to Europe and an outward-looking trade policy, she told a business forum organised by the news group Bloomberg.

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May then took 20 minutes of largely unchallenging questions from Ginni Rometty, the chief executive of IBM. But for the final question Rometty passed the microphone to Schwarzman, who posed a tougher challenge.

Asking about “downside management”, he said Blackstone had bought a new company in the UK last week, and thus had some faith in Brexit.

“But on the other side, things could really go off with a bad Brexit and also a change of government. The thing that we really worry about is: how bad can things get?”

He added: “We believe in the good things. But in terms of just thinking about it from a risk management perspective it’s a little daunting for those of us on the outside.”

Last week, Blackstone paid £1.5bn for thousands of commercial spaces underneath Victorian railway arches in the UK.

May responded by seeking to reassure Schwarzman and a dozens of other business leaders at the venue in New York, where she is for two days for the UN general assembly.

“I absolutely appreciate and understand that – you’re making decisions and looking ahead to what the future holds,” she said. “And at the moment there is that uncertainty about what the future will hold in terms of Bexit.”

May continued: “First of all … we believe that we will and can get a good deal. And that’s because it’s not just about the UK, it’s about the EU as well. And I think that continued trading relationship is going to be good for both sides.”

There were still, she added, “several weeks of intense work to be done” to seal a deal, arguing that Brexit brought “a sharper focus” to existing efforts to reform the UK economy.

“That’s why we’ve been looking at this concept of the economy of the future, to make sure we have those building blocks in place that are needed post-Brexit so that when we leave the EU we will be able to ensure that we are still a great country to do business in.”