The think tank behind the pro-US free trade plan launched this week by leading Brexiteers went on a “lucrative” American tour to raise money for its Brexit work.

The Institute of Economic Affairs (IEA) and its trade policy chief, Shanker Singham, launched the “Plan A+” report in London on Monday with the backing of Boris Johnson and David Davis, as well as the European Research Group, an influential faction of Eurosceptic MPs led by Jacob Rees-Mogg.

An Unearthed investigation this summer revealed that IEA boss Mark Littlewood had been on what he described as a “lucrative” US tour in February to raise funds for the IEA’s new trade policy unit.

The investigation also showed the IEA offering to broker access to senior ministers for an undercover reporter posing as representative of US agribusiness investors. IEA director Mark Littlewood said the think tank, which is registered as an educational charity, was in the “Brexit influencing game”.

The high-profile Plan A report has been widely touted as the arch-Brexiteers’ alternative to Theresa May’s Chequers plan. It calls for the UK to move away from the EU’s strict regulations to “capture the Brexit Prize” and strike free-trade deals with other countries, particularly the US.

This could see the UK forced to loosen environmental standards and accept controversial products such as chlorine-washed chicken and hormone-treated beef.

Sticking to EU rules for agri-food, as the Chequers plan demands, “means that the UK is severely constrained in its ability to change rules in the area of goods and agri-food, as well as in certain horizontal areas such as labour, and the environment,” the IEA report says.

An IEA spokeswoman declined to reveal who had funded the report, saying: “We respect the privacy of our donors and do not place a list of them in the public domain.”

She added that IEA reports are independent of their funders, saying: “We make independent editorial decisions and then seek funding. Any received funding does not, under any circumstances, influence the conclusions of reports and our rigorous peer-review process means we are confident that our output is independent and free from conflict of interest.”