US President Donald Trump has said his administration wants to accept a plane worth an estimated $400m (£303m) as a gift from Qatar, calling it “a great gesture” that he would be “stupid” to turn down.

The potential move has been labelled “wildly illegal” by some members of the rival Democratic Party – something the White House denies – and it has attracted criticism from some of Trump’s supporters.

Qatar itself earlier said the reports about the plane were “inaccurate”, and that negotiations were continuing.

The news comes as Trump visits several countries in the Middle East, including Qatar.

BBC Verify has been looking into the legality of presidents accepting gifts.

On Sunday, US media reported that the Trump administration was preparing to accept a Boeing jumbo jet from the Qatari royal family – saying that the plane would be refitted and used temporarily as Air Force One, the name for the plane used by presidents.

Trump later posted on Truth Social: “The Defense Department is getting a gift, free of charge, of a 747 aircraft to replace the 40-year-old Air Force One, temporarily, in a very public and transparent transaction.”

When questioned by reporters, Trump said: “It’s a great gesture from Qatar. I appreciate it very much. I would never be one to turn down that kind of an offer.”

In February, Trump said he was “not happy with Boeing” about delays to two new Air Force One jets that he is expecting to receive directly from the firm. He added that the White House could instead “buy a plane or get a plane, or something”.

The Qatari plane was pictured in Palm Beach, Florida, in February where Trump inspected it. It is currently fitted with three bedrooms, a private lounge and an office, according to its specification summary document from 2015.

A Qatari official has told CNN the plane is being given from the Qatari defence ministry to the Pentagon, and that it will be modified to meet Air Force One’s safety and security standards.

Experts say this is likely to take years, which means the plane may not be ready for use until near the end of Trump’s term.

Trump has said the plane will go directly to his presidential library after he leaves office, and that he “wouldn’t be using it” after his presidency.

Nonetheless, the move has led to criticism from Democrats as well as some long-time Trump supporters, including Laura Loomer who said: “This is really going to be such a stain on the admin if this is true.”

Several senior Democrats have claimed that accepting the gift would be illegal.

Democratic Senator Adam Schiff quoted a section of the US Constitution that said no elected official could accept “any present… of any kind whatever” from the leader of a foreign state without congressional approval.

Frank Cogliano, a professor of American history at the University of Edinburgh, says this clause “was intended to prevent bribery to influence the government”.

“It is certainly stretching the Constitution and we have not seen a gift on this scale, or of this nature”, says Professor Andrew Moran, a constitutional law expert at London Metropolitan University.

There have been a number of other laws passed by Congress relating to the acceptance of foreign gifts, such as the Foreign Gifts and Decorations Act of 1966, which means that congressional consent is required for the acceptance of foreign gifts above a certain value.

Currently US officials can accept gifts valued at less than $480 (£363).

Although Trump has referred to the plane ultimately going to his “library”, experts have suggested he really means his museum foundation.

Ex-presidents typically have a library housing their archive of documents, and a museum – typically funded by private donations – full of memorabilia and open to the public.

Experts who BBC Verify spoke to said the fact that the plane could be given to the administration – and not to the president directly – before being transferred to his museum, may not get around the potential violation of the constitution.

Jordan Libowitz – from the organisation Citizens for Responsibility and Ethics in Washington – said any use of the plane by Trump after leaving office would cross a line: “Reagan’s Air Force One ended up in his presidential library, but there’s a difference there. The plane was decommissioned, Reagan never flew on it again, and it sits inside as a museum piece.”

The US Justice Department has reportedly drafted a memo explaining why it thinks accepting the jet would be permissible, although this has not been made public.

When the White House Press Secretary, Karoline Leavitt was asked about the legality of the deal, she said: “The legal details of that are still being worked out, but of course, any donation to this government is always done in full compliance with the law.”

President Trump is on a four-day trip to Saudi Arabia, Qatar and UAE, hoping to drum up investment for the US.

His visit follows a series of business deals announced by the Trump Organization, which is run by the president’s sons, Eric and Donald Jr.

These include plans to build golf courses and luxury villas in Qatar and the UAE.

President Trump is not currently affiliated with the Trump Organization, having handed over management responsibilities to his children after entering the White House on 20 January.

A deal was announced by the Trump Organization at the beginning of May to develop a Trump-branded luxury 18-hole golf course and a collection of luxury villas north of Qatar’s capital, Doha.

At the time, Eric Trump said: “We are incredibly proud to expand the Trump brand into Qatar through this exceptional collaboration with Qatari Diar and Dar Global.”

Dar Global is a publicly owned Saudi construction company; Qatari Diar is a Qatari state-owned company.

Separately, on 30 April, the Trump Organization announced it would build “the region’s first Trump International Hotel & Tower” in the “heart of Dubai” consisting of 80 floors of “luxury living and world-class hospitality”.

Eric Trump also visited the UAE recently, speaking at Token 2049, a cryptocurrency conference, on 1 May.

Asked if Trump was likely to meet anyone involved in the family business during his trip, White House Press Secretary Karoline Leavitt said it was “ridiculous” to suggest the president was doing anything for his own benefit.