The Labour party has called for a parliamentary investigation into the former cabinet minister Owen Paterson after it emerged unknown donors had funded his trips to the tune of nearly £39,000.

The Guardian reported earlier this week that the funds were routed through his personal thinktank, UK 2020, of which he is chairman and sole director.

MPs are required to publicly declare any donor who pays for an overseas visit costing more than £300 in the register of members’ interests. However, as a limited company, UK 2020 is not required to disclose its donors.

By declaring the thinktank as the “donor” behind the visits, Paterson has effectively avoided revealing who actually funded them. He says he has declared all required information in his register of interests.

In a letter to Kathryn Stone, the parliamentary commissioner for standards, the shadow Cabinet Office minister, Jon Trickett, said it must “immediately be determined” whether Paterson’s conduct had broken the rules governing the conduct of MPs.

“Paterson appears to be both the recipient of donations and the controlling intermediary through which they are paid,” Trickett’s letter stated. He said without information on the true source of donations, “the register of members’ interests is unable to fulfil its vital purpose”.

Trickett said the purpose of the register was to make public any private interest that might reasonably be thought to have influenced an MP’s actions or behaviour.

“Without knowing the true source of UK 2020’s funding, Paterson’s declaration of trips as being funded by his thinktank does not provide the clarity needed to make such a judgment,” he wrote.

“This could constitute a serious breach of the rules. To put it simply: where does the money come from?”

My letter to the Parliamentary Commissioner for Standards, calling for an investigation into £39,000 of donations made to Owen Paterson MP, from the think-tank he himself set up and is director of. Where does the money come from? pic.twitter.com/NepFRfd3R2 — Jon Trickett (@jon_trickett) January 24, 2019

Trickett also warned failure to investigate and act on Paterson’s use of his private company as an intermediary would pave the way for all other members of parliament to set up their own shell companies through which to receive donations.

“A worrying precedent would be established whereby it is acceptable for politicians to receive donations via intermediary companies that they themselves establish and direct, and because of this are under no obligation to disclose the true source of donations,” he wrote.

Paterson was appointed environment secretary by David Cameron in 2012, despite disputing the scientific consensus on climate change. He stepped down two years later and set up UK 2020 in October 2014. He began taking trips he said were funded by the thinktank in early 2015.

His last three UK 2020-funded trips were all to Washington DC, where he gave speeches demanding a hard Brexit or campaigning against Theresa May’s withdrawal deal. The three trips had a combined value of almost £19,000.

In October 2017, he gave a speech to the Competitive Enterprise Institute, a rightwing US thinktank, attacking environmental regulations and campaigners, and demanding the UK abandon the “precautionary principle”, which requires new technologies to be proved safe before they can be widely used.

In September 2018 he gave a speech to the Heritage Foundation, another rightwing thinktank, criticising May’s so-called Chequers proposal for negotiating a withdrawal deal with the EU. Two months later, he returned and participated on a panel calling for the UK to crash out of the EU without a deal.

Paterson has been approached for comment. He previously said: “All the expenses incurred on these trips have been declared according to parliamentary rules.”