Former US President Donald Trump has sued New York Attorney General Letitia James, seeking to end her civil investigation into his business practices.

In the lawsuit, filed two weeks after Ms James requested that Mr Trump sit for a January 7 deposition, Trump alleges that the investigation has violated his constitutional rights in a “thinly-veiled effort to publicly malign Trump and his associates”.

“Her mission is guided solely by political animus and a desire to harass, intimidate, and retaliate against a private citizen who she views as a political opponent,” the former president’s lawyers wrote in the suit.

Mr Trump, a Republican, seeks a permanent injunction barring Ms James, a Democrat, from investigating him and a declaratory judgment stating that she has violated his rights.

Messages seeking comment were left with Ms James’s office and Mr Trump’s lawyers. News of the lawsuit, filed in federal court in Albany, was first reported by The New York Times.

Ms James has spent more than two years looking at whether the former president’s company, the Trump Organisation, misled banks or tax officials about the value of assets — inflating them to gain favourable loan terms or minimising them to reap tax savings.

Ms James’s investigators last year interviewed one of Mr Trump’s sons, Eric Trump, who is an executive of the company, as part of the investigation.

Her office went to court to enforce a subpoena on the younger Mr Trump and a judge forced him to testify after his lawyers abruptly cancelled a previously scheduled deposition.

Ms James’s request for the former president’s testimony, first reported on December 9, was the first step in a process that will now probably lead to issuing a subpoena and going to a judge to order him to cooperate if he were to refuse.

It is rare for law enforcement agencies to issue a civil subpoena for testimony from a person who is also the subject of a related criminal investigation. That is partly because the person under criminal investigation could simply cite their Fifth Amendment right to remain silent.

It is unlikely that Trump’s lawyers would allow him to be deposed unless they were sure his testimony could not be used against him in a criminal case.

The Manhattan District Attorney’s office is conducting a parallel criminal investigation into Mr Trump’s business dealings. Although the civil investigation is separate, Ms James’s office has been involved in both.

Earlier this year, Manhattan District Attorney Cyrus Vance Jr. gained access to the real estate mogul’s tax records after a multi-year fight that twice went to the US Supreme Court.

Mr Vance, who is leaving office at the end of the year, recently convened a new grand jury to hear evidence as he weighs up whether to seek more indictments in the investigation, which resulted in tax fraud charges in July against the Trump Organisation and its long-time CFO Allen Weisselberg.

Mr Weisselberg pleaded not guilty to charges alleging that he and the company evaded taxes on lucrative fringe benefits paid to executives.

Both investigations are at least partly related to allegations made in news reports and by Mr Trump’s former personal lawyer, Michael Cohen, that he had a history of misrepresenting the value of assets.

Ms James’s office issued subpoenas to local governments as part of the civil investigation for records pertaining to Mr Trump’s estate north of Manhattan, Seven Springs, and a tax benefit he received for placing land into a conservation trust. Mr Vance later issued subpoenas seeking many of the same records.

Ms James’s office has also been looking at similar issues relating to a Trump office building in New York City, a hotel in Chicago and a golf course near Los Angeles. Her office also won a series of court rulings forcing his company and a law firm it hired to turn over troves of records.

Ms James had announced a run for New York governor in late October, but earlier this month she suspended that campaign and cited ongoing investigations in her decision to instead seek re-election as state attorney general.