The US has accepted a plane intended for the Air Force One fleet from Qatar, a gift that has sparked criticism including from some of President Trump's biggest supporters.

"The secretary of defense has accepted a Boeing 747 from Qatar in accordance with all federal rules and regulations," Chief Pentagon spokesman Sean Parnell said in a statement on Wednesday.

The plane will need to be modified before it can be used as part of Air Force One - the president's official mode of air transport.

The White House insists that the gift is legal, but the announcement of the transfer a week ago caused huge controversy.

The plane is a gift from the Qatari royal family and is estimated to be worth $400m (£300m). The White House says that the new plane will be transferred to Trump's presidential library at the end of his term.

It could require years to fit with additional security systems and upgrades required to carry the president - including the ability to withstand the electromagnetic pulse from a nuclear blast, and to refuel mid-flight.

Mark Cancian, a senior adviser with the Center for Strategic and International Studies' Defense and Security Department, says the costs of such retrofitting could easily run to $1bn (£750m).

Justifying the transfer a week ago Trump said: "They're giving us a gift". The president has also said it would be "stupid" to turn down the plane.

The US Constitution has a provision known as the Emoluments Clause, which prohibits gifts to public officials from foreign governments without permission of Congress. The transfer has not received congressional approval.

The president has argued that the plane transfer is legal because it is being given to the US defence department, and not to him personally. He also insisted he would not use it after leaving office.

The current Air Force One fleet includes two 747-200 jets which have been in use since 1990, along with several smaller 757s.

Trump has expressed his displeasure at the aircraft manufacturer Boeing, which has been contracted to provide the White House with two 747-8s directly.

His team negotiated to receive them during his first term in office, though there have been repeated delays and Boeing has cautioned that they will not be available for two or three more years.

Trump surreptitiously visited the Qatari plane in Palm Beach, near his Mar-a-Lago resort, just a few weeks after the start of his second term in office.

The president insists there is no quid-pro-quo involved and that the plane is a simple exchange between two allies.

On Truth Social he wrote: "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."

Qatari Prime Minister Sheikh Mohammed bin Abdulrahman bin Jassim Al-Thani has said the transfer "is a government-to-government transaction.

"It has nothing to do with personal relationships - neither on the US side, nor the Qatari side. It's between the two defence ministries," he said.

But those assertions have done little to calm the criticism of the deal, including from a number of Trump's allies in Congress and the right-wing media.

"I think it's not worth the appearance of impropriety, whether it's improper or not," Rand Paul, Republican senator from Kentucky, told Fox News.

"I wonder if our ability to judge [Qatar's] human rights record will be clouded by the fact of this large gift," Paul said.

Another Republican senator, Ted Cruz of Texas, said accepting the gift would pose "significant espionage and surveillance problems".