The Trump administration has blocked the US ambassador to the European Union from appearing before a congressional impeachment investigation.

Gordon Sondland was due to meet behind closed doors on Tuesday with staff from three Democratic-led House committees.

The committee chairmen vowed to issue a legal summons to Mr Sondland.

Mr Sondland was set to be grilled about any role he played in prodding Ukraine to investigate President Donald Trump’s political rival, Joe Biden.