Visit to Downing Street in May last year – to discuss trade and investment – was not recorded by PM on official list of meetings

David Cameron hosted a Thai billionaire whose conglomerate is at the centre of Guardian revelations of fishing industry slavery, but did not record the visit on his official list of meetings.

The prime minister was photographed with Dhanin Chearavanont, chairman and chief executive of Charoen Pokphand (CP) Group, at Downing Street in May last year, as representatives of the company came to Number 10 to discuss trade and investment opportunities with officials.

A Guardian investigation has established that one of CP Group's subsidiaries, CP Foods, feeds the prawns that it exports to the UK and US with fishmeal sourced from "ghost ships" manned with slaves.

The encounter between CP Group executives and Cameron does not appear on the official list of the prime minister's engagements. However, it was revealed on the company's website in June 2013 and is recorded on a separate list for the Foreign Office minister Hugo Swire, who listed the purpose of seeing CP Foods in May 2013 as "meeting with the PM".

A Number 10 spokesman said: "CP Foods met with No 10 officials to discuss trade and investment opportunities." A source said Cameron had not been scheduled to meet the Thai businessmen, who were invited for a discussion with his aides.

Following the disclosures that CP Foods supplied British supermarkets including Tesco, Morrisons, Aldi, Iceland and Co-op, Cameron's spokesman said on Wednesday it was up to consumers whether they chose to eat prawns that had been produced through the work of slaves.

Asked whether supermarkets should stop stocking seafood linked to forced labour, the No 10 official said: "Decisions that consumers make, they are for individuals … Consumer standards and retail standards and social responsibility is often driven by consumers and rightly so."

Bob Miller, CP Foods' UK managing director, told the Guardian this week: "We're not here to defend what is going on. We know there are issues with regard to the [raw] material that comes in [to port], but to what extent that is, we just don't have visibility."

Labour has called on the government to turn up the pressure on UK retailers to shun products linked to forced labour, including requiring companies to declare any use of slavery in their supply chains. However, the government appears to favour a voluntary approach for now.

The Queen's speech included a bill to bring in penalties to deter modern slavery, but it has been criticised for its narrow focus on slavery in the UK and failure to tackle the problem of goods produced through slavery abroad.

A Home Office spokeswoman said slavery was not a problem that could be solved by legislation alone or by the end of this parliament. "It needs the engagement and long-term commitment of all sections of society," she said. "Companies have a social responsibility to ensure that those they do business with are not involved in the exploitation of others. If businesses take no action they risk both their reputation and profit.

"Today, the home secretary and minister for modern slavery, Karen Bradley, have hosted a roundtable with key business leaders to look at how the government can work with businesses to eliminate forced labour and exploitation from supply chains."