image copyrightGetty Imagesimage captionThomas Barrack speaking at the Republican National Convention in 2016

Billionaire and long-time Donald Trump ally Thomas J Barrack has been arrested in Los Angeles for allegedly acting as an agent of a foreign government.

The 74-year-old investment firm founder served as a key adviser to the former US president's 2016 campaign, and was considered a top fundraiser.

Mr Barrack is accused of illegally lobbying on behalf of the United Arab Emirates during and after the campaign.

A spokesman for Mr Barrack said he planned to plead not guilty.

He is the latest ex-Trump official to face federal charges.

Mr Barrack is charged with conspiracy, obstruction of justice and making multiple false statements to the FBI during a 2019 interview.

Also charged are Matthew Grimes, 27 – who works for Mr Barrack – and UAE citizen Rashid Sultan Rashid Al Malik Alshahhi, 43, according to the seven-page indictment.

Acting Assistant Attorney General Mark Lesko said the conduct alleged in the indictment was "nothing short of a betrayal" of US officials including Mr Trump himself.

The three men are accused of seeking to advance the interests of the UAE government both by influencing Trump officials and through media appearances.

In May 2016 Mr Barrack is alleged to have inserted language praising the UAE into a Trump campaign speech. An advanced draft of the speech was allegedly sent to Mr Alshahhi to pass on to UAE officials.

The defendants are also alleged to have received "talking points" from senior UAE officials for use in media appearances.

"After one appearance in which Barrack repeatedly praised the UAE, Barrack emailed Alshahhi, 'I nailed it… for the home team,' referring to the UAE," the charging document states.

The men also allegedly agreed to promote the candidacy of a UAE-favoured individual to serve as US ambassador to Abu Dhabi.

In December 2016 – following the election – Mr Barrack is also alleged to have advised Mr Grimes and Mr Alshahhi to create a "wish list" of US foreign policy items that the UAE wanted to see accomplished during the Trump administration.

While Mr Barrack and Mr Grimes were both arrested in California on Tuesday morning, Mr Alshahhi remains at large.

With the announcement, Mr Barrack becomes the latest Trump aide to face criminal charges, including former campaign chairman Paul Manafort and former Trump Organization lawyer Michael Cohen.