The former health secretary Matt Hancock was given an overnight stay at a country estate owned by the head of Randox, the healthcare firm that had hired the MP Owen Paterson as a consultant.

During a two-day visit to Northern Ireland as health secretary in 2019, Hancock had a private dinner and stayed overnight at the Dundarave country estate in County Antrim, which is owned by Peter Fitzgerald, Randox’s founder.

The overnight stay was disclosed in documents released under the Freedom of Information Act. It was not included in the official register of hospitality received by ministers.

Through a spokesperson, Hancock said he did not need to declare the hospitality as he had not accepted it in a ministerial capacity. However, transparency campaigners disagreed and said there was a “clear expectation” that ministers should declare such hospitality and follow the spirit of the rules.

Questions have been raised about the relationship between Randox and the Conservative party after the firm was awarded almost £500m of public funds during the Covid pandemic for testing.

Randox also hired Paterson as a consultant, paying him £100,000 a year. Paterson resigned from parliament last year after he exploited his position as an MP to lobby for his clients, including Randox.

Randox donated £160,000 to the Conservative party between 2010 and 2018. Paterson directly lobbied Hancock on behalf of Randox during the pandemic. After Paterson’s lobbying, Hancock chased his officials, saying he was “very worried” about how his department was treating Randox and other firms.

During the Northern Ireland visit in 2019, Hancock met three firms, including Randox, that at that time were paying Paterson to be their consultant. Official documents obtained by Transparency International UK under the Freedom of Information Act suggest Paterson was partially involved in organising Hancock’s visit.

They also show how Hancock was invited by Randox to have dinner and stay overnight at Fitzgerald’s Dundarave estate during his visit.

The County Antrim estate is described as “magnificent … in a glorious setting” with “a fine Italianate mansion at its heart.”

On 21 March 2019, Hancock toured Randox’s laboratories in Belfast. An email sent to the health department, which appears to have been written by a Randox employee, said: “Understand SoS [secretary of state] will be with us, Randox Science Park … Then SoS goes to other visits before joining us for a private dinner and overnight that evening.”

On the same day, Hancock carried out a number of other official visits, including at Ulster hospital. He also attended a forum to discuss health and food. Two of the speakers were from Devenish, a firm that makes animal feed, and Lynn’s Country Foods, a firm that makes meat products.

The two firms were at that time paying Paterson a total of £61,000 a year to be their consultant.

The rules governing the conduct of ministers, known as the ministerial code, state that when politicians accept hospitality in a ministerial capacity, details should be made public through a register that is published regularly. Official guidance identifies dinners given by companies as the type of hospitality that should be declared if received in a ministerial capacity.

Hancock’s spokesperson, James Davies, said there had been no need to declare the private dinner and overnight stay at the Randox founder’s estate because it was political rather than departmental.

“Everything was declared properly and appropriately,” Davies said, adding that the relevant paragraphs of the ministerial code were “a departmental responsibility. If they judge an event political, then this doesn’t apply. Perhaps you should be speaking to [the Department of Health and Social Care], rather than Mr Hancock.”

He continued: “Staying overnight is absolutely fine. It was a political dinner, and Mr Hancock met many [Northern Ireland] politicians including Robin Swann, another relationship that became critical in responding to the pandemic. It’s absurd to say there was anything wrong with this.”

That interpretation of the rules was disputed by Rose Whiffen, a research officer at Transparency International UK, which obtained the documents. She said: “When ministers accept hospitality, especially from political donors, there is a clear expectation this should be declared and a matter of public record.

“When a secretary of state is uncertain over whether they have to report being wined and dined under the ministerial code, it is always better for them to err on the side of caution and follow the overriding spirit of the rules by doing so.”