US president Donald Trump is being investigated over a potential conflict of interest involving increased US military spending at Prestwick Airport and visits to his Turnberry resort nearby.

The United States Congress has launched a probe into whether increasing expenditure at the airport and allegations of cut-price deals at Trump’s Ayrshire golf resort offered to visiting US military personnel is a violation of the American constitution.

The debt-ridden airport has been fighting off closure and is said to be integral to the Trump business, which is also running at a loss.

Chairman of the Congress oversight committee Elijah Cummings wrote to then-acting secretary of defence Patrick Shanahan in June it has emerged, to raise “serious conflict of interest concerns” about Trump making money from military trips to Scotland.

It followed an investigation by The Scotsman newspaper which detailed how Prestwick’s parent company has received more than £9 million for 644 orders to refuel US Armed Forces aircraft between October 2017 and March this year.

The paper claims in the six months since, further refuelling orders have made Prestwick an additional £4.8m.

The June letter reveals concerns that US military spending at Prestwick – the closest airport to Trump’s resort – appears to have “increased substantially since the election”.

Following reports of the airport offering “cut-price rooms for select passengers and crew” and free rounds of golf at Turnberry for US military staff and civilian air crews, Trump has been warned his financial stake in the resort raises questions about the increase in airport spending.

Both Trump Turnberry and the airport have operated at significant losses in recent years. The Scottish Government bought the airport, which is 20 miles (30km) north of Trump Turnberry, for £1 in 2013, when it was facing closure. In June, it was put up for sale but no buyer has been found.

The committee is demanding to see all the financial documents from the US Department of Defence relating to Trump Turnberry, in addition to details of all communications between the two sides. But reports from the US suggest there has yet to be any response.

Cummings’s letter reads: “Two years before the 2016 election, President Trump spent hundreds of millions of dollars to purchase and renovate the Turnberry golf course in Scotland. To date, the property has continued to suffer financial losses and has not turned a profit for the president of his companies.

“The airport closest to the Trump Turnberry golf course – Glasgow Prestwick Airport – has been viewed as integral to the golf course’s financial success, yet it too has lost millions of dollars every year since its purchase by the Scottish Government in 2013.”

Citing Defence Logistics Agency records showing 629 fuel purchase orders worth $11m since October 2017, the letter adds: “Given the president’s continued financial stake in his Scotland golf courses, these reports raise questions about the president’s potential receipt of US or foreign government emoluments in violation of the US Constitution and raise other serious conflict of interest concerns.”

The Scottish Government faced a backlash in 2017 after the Turnberry resort received a tax rebate. Later that year, the rules changed and it no longer qualified.

A spokesperson said: “Glasgow Prestwick Airport is operated on a commercial basis and at arm’s length from the Scottish Government, in compliance with European Union state aid rules. Ministers do not intervene in the commercial discussions at the airport.”