The tech company run by the US businesswoman whose friendship with Boris Johnson has sparked a conflict of interest row hastily set up a new office after it emerged it was facing an investigation over its eligibility for a six-figure taxpayer-funded grant.

Jennifer Arcuri’s company Hacker House was awarded a £100,000 cyber-skills grant intended for UK-operating businesses in January even though she and her business partner husband had relocated to California last year.

The digital minister, Matt Warman, told MPs on Wednesday that £53,000 of the grant yet to be paid to the company would be frozen until the government had completed its review, which it launched this week.

Now it has emerged that on Wednesday the company re-registered its address with Companies House from an old rented flat in Macclesfield, where Arcuri and her husband stayed before moving to the US in 2018, to a “virtual office” in central London where none of its staff appear to work.

Companies House confirmed that it did not receive the online application for Hacker House’s address change until 4.10pm – meaning it was made hours after MPs quizzed Warman in the Commons on the company’s eligibility for the grant.

The central London building where Hacker House has registered its company address. Photograph: Simon Murphy/The Guardian

When the Guardian visited the Grade II-listed office, based above a Marks & Spencer on Fleet Street overlooking St Paul’s Cathedral, and asked to speak to someone from Hacker House, a reporter was told by a receptionist: “That’s a virtual office so they’re not actually based here.”

Asked how long the company had been using the address, the receptionist said: “They’re fairly new … [they registered] I think just a couple of days ago.”

The virtual office, which is operated by Regus, could not yet receive post for Hacker House as it had not verified its account, she said. The Companies Act 2006 states: “A company must at all times have a registered office to which all communications and notices may be addressed.”

Regus’s website says clients can pay for a virtual office in Fleet Street for £178 a month, enabling them to have a “prestigious business address”. It states: “Use the address on your company documents, website, business cards or web listings. We will handle your incoming mail, which you can collect for free or choose from a daily, weekly or monthly mail forwarding package for one simple additional monthly cost. Local phone number and live professional receptionist to answer your calls in your company’s name.”

Sign outside the registered address for Hacker House on Thursday. Photograph: Simon Murphy/The Guardian

Warman said in the Commons on Wednesday that the Department for Digital, Culture, Media and Sport (DCMS) had done the “usual due diligence” on Hacker House, adding that it had “a British phone number”. But when the Guardian rang the UK number on Wednesday it got through to a woman who said she was based in the US.

Arcuri’s husband and co-director, Matthew Hickey, claimed on Twitter that the company employed “several British people”, adding: “There’s nothing improper about it and we have been regularly updating DCMS and working with UK government on its needs.”

Companies House records state that Arcuri and Hickey are both directors of the company, which was set up in 2015. Hacker House, which offers cybersecurity training, states on its website: “Taught by world-renowned ethical hackers, our students are plunged head-first into real world cyber-attack simulations and are assessed across a variety of practical assignments designed to cover every angle. In short, we teach students how to think, act and move like a hacker – and then outmanoeuvre them.”

The new office is the latest twist in a brewing controversy over Johnson’s involvement with Arcuri, 34, whose companies have been awarded more than £126,000 of public money.

Her other business, Innotech, was handed more than £26,000 of public funds while Johnson was London mayor, prompting questions over whether the company received preferential treatment because of her friendship with him. Arcuri was allowed to attend three overseas trade missions led by Johnson while he was mayor.

It has also emerged that in 2016 Arcuri admitted that Hacker House employed Lauri Love, who is wanted by US authorities for allegedly hacking into websites. Arcuri told Love’s extradition hearing in 2016 that she had met him the previous year and that he worked for the company Thursday to Monday.

Johnson has stonewalled repeated questions about his friendship with Arcuri, first revealed by a Sunday Times investigation.

Hacker House has been contacted for comment. Arcuri has previously said: “Any grants received by my companies and any trade mission I joined were purely in respect of my role as a legitimate businesswoman.”

A Companies House spokesman said: “All companies must provide Companies House with a registered office address. The address will appear on the register and be used for official communications, such as letters from Companies House.”