Mr Trump refuses to share documents concerning his fortune and business.

His lawyers argue he enjoys total immunity while in office.

The ruling will test that claim and has implications on how far US lawmakers can scrutinise the president.

Even a ruling in Congress's favour would not necessarily make Mr Trump's tax returns public before his bid for re-election in November.

Mr Trump, who made his money as a property developer, is the first president since Richard Nixon in the 1970s not to have made his tax returns public.

He calls the investigation into his tax affairs a "witch hunt" and sees the congressional case as a device to harass him politically.

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