The White House has officially refused to co-operate with the impeachment inquiry against President Donald Trump.

Read alsoYovanovitch is latest casualty of Trump war on career diplomats – media

A letter sent to Democratic leaders rejected it as "baseless" and "constitutionally invalid," according to the BBC.

Three Democratic-led House committees are investigating Mr. Trump.

The inquiry is trying to find out if the president held back aid to Ukraine to push it to investigate Joe Biden, who is running for the Democratic presidential nomination.

The White House letter comes hours after the Trump administration blocked the U.S. ambassador to the European Union from appearing before a congressional impeachment investigation.