Donald Trump must obey Congress by releasing eight years' worth of tax returns, a federal appeals court has ruled.

Judges on Wednesday denied for the second time the US president's attempt to stop his accounting firm from releasing the financial records to the House oversight committee.

Jay Sekulow, Mr Trump's lawyer, said the president would appeal to the US Supreme Court.

The 8-3 vote by the US Court of Appeals for the DC Circuit, declining Mr Trump's request to rehear arguments that the subpoena to Mazars LLP was illegitimate, brings Democrats closer to shedding light on his business interests and how he built his fortune.

The committee subpoenaed Mazars this year, saying it needed the records to determine whether or not Mr Trump complied with laws requiring disclosure of his assets, and to assess whether those laws needed to be changed.

While campaigning for the presidency in 2016, Mr Trump broke with a decades-old convention of candidates releasing their tax returns to the public.

Mr Trump sued the House panel in April, arguing that its subpoena exceeded limits on Congress' investigative power.

He said the true motive for the subpoena was to expose private financial information "with the hope that it will turn up something that Democrats can use as a political tool against the president".

A lower court judge ruled against Mr Trump in May, saying the documents might assist Congress in passing laws and performing other core functions.

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The May decision was the first time a federal court waded into the tussle about how far Congress can go in investigating Mr Trump and his business affairs, and marked an important victory for House Democrats.

A three-judge panel of DC circuit judges, in a 2-1 ruling, upheld the lower court judge in October.

"Contrary to the president's arguments, the committee possesses authority under both the house rules and the constitution to issue the subpoena, and Mazars must comply," Judge David Tatel wrote on behalf of the majority.

Judge Neomi Rao, who was appointed by Mr Trump to the DC appeals court, dissented from the October decision.

Ms Rao and another Trump appointee to the court, Gregory Katsas, voted to rehear the case, Wednesday's order showed. They were joined by Karen Henderson, an appointee of former President George H W Bush.

The ruling over Mr Trump's tax records came the same day the chief US diplomat in Ukraine gave explosive new testimony to House investigators about the president's attempts to solicit foreign assistance in the 2020 presidential campaign.

William Taylor, the acting US ambassador to Ukraine, said an aid had revealed they overheard a phone call between Mr Trump and EU ambassador Gordon Sondland in which the president apparently asked about his requested Ukrainian investigations into political rivals.