The US state department inspector-general fired late last week was investigating the Trump administration’s decision to bypass Congress and fast-track a multibillion-dollar arms sale to Saudi Arabia, a senior Democrat has claimed.

Eliot Engel, the Democratic congressman from New York and chair of the House foreign relations committee, said on Monday that the arms sale “may be another reason” for the US president’s abrupt sacking of Steve Linick, inspector-general of the US Department of State.

Notifying Congress of his decision late on Friday night, Mr Trump said he no longer had “the fullest confidence” in the federal watchdog. A senior White House official said at the weekend that Mike Pompeo, US secretary of state, recommended the move, and Mr Trump agreed.