WASHINGTON (AFP) - US Treasury Secretary Steven Mnuchin and Scottish actress Louise Linton will marry on Saturday (June 24) in a glitzy Washington affair, with President Donald Trump and First Lady Melania set to be on hand to toast the newlyweds.

The wedding, to be held in a historic auditorium near the National Mall, had been a closely held secret until a friend of the couple, US Commerce Secretary Wilbur Ross, let word slip earlier this week.

About 300 guests are expected to attend, including Vice-President Mike Pence and other members of the Trump administration. Canadian Finance Minister Bill Morneau is expected to be among the guests.

Linton, 36, holds journalism and law degrees. She has appeared on American television shows including CSI: NY and Cold Case, and had small roles in several movies, according to her website.

She also works as a film producer.

A book Linton wrote about spending her gap year in Zambia, In Congo's Shadow, drew criticism for inaccuracies and for a tone that a Washington Post critic described pointedly as "the perfect White-Saviour-in-Africa story." She later apologised and agreed to give any profits to charity.

Linton met Mnuchin at a wedding reception in Los Angeles in 2013, and he proposed two years later, she told The New York Times.

Mnuchin, 54, a former Goldman Sachs executive and hedge-fund manager, is also a former Hollywood film financier.

He had named Linton last month as interim head of Dune Entertainment, a company he helped found, but she resigned after Democrats raised questions about possible ethical conflicts.

Mnuchin has been a key player in Trump administration efforts to craft and pass tax reforms, and in urging Congress to raise the national debt ceiling.

This will be Mnuchin's third marriage and Linton's second.