US President Donald Trump has signed an order designating the Oval Office as a national monument, a move intended to prevent future presidents from making structural alterations to the historic presidential workspace.

Trump signed the proclamation on Thursday, extending the protection to the adjoining Cabinet Room and declaring that neither space would be demolished, altered or subjected to future construction work.

Speaking during the signing ceremony at the White House, Trump said the decision would “make it impossible for anybody ever to destroy this incredible space.”

“It won’t be able to be taken down, destroyed, touched, or anything else. It’s called actually a national monument,” he told reporters.

The development comes amid growing criticism of Trump's attention to White House renovation projects following significant losses suffered by his Republican Party in the midterm elections.

Critics have accused the president of overlooking pressing public concerns, particularly the Iran war and the rising cost of living, while concentrating on construction and beautification projects in Washington.

A day after Democrats secured victories in both the House of Representatives and the Senate, Trump largely sidestepped the election outcome and instead showcased ongoing construction work on his proposed White House ballroom.

His latest move focused on the Oval Office, which he has extensively decorated with gold ornaments since returning to the White House last year.

The proclamation has also sparked speculation about possible further renovations to the West Wing, following Trump's previous suggestions that journalists could be moved out of the historic briefing room located close to the Oval Office.

The signing ceremony was not broadcast by major US television networks after Trump excluded CNN from the rotating pool of television organisations covering White House events.

However, Trump appeared to make a reference to the midterm elections by placing a red baseball cap bearing the word “veto” on the Resolute Desk during the ceremony.

The 80-year-old Republican has threatened to use his presidential veto powers to block legislation from Democrats, although he has also indicated his willingness to work with the opposition party.