LONDON: Boris Johnson threw Britain's ambassador to Washington "under the bus" when he refused to back the envoy in his row with Donald Trump, junior Foreign Office minister Alan Duncan said on Wednesday.

Showing the anger within his department at the treatment of Kim Darroch, Duncan told the BBC Johnson had failed to back Darroch during a televised debate late on Tuesday in order to serve his own personal interests.

Johnson is the favourite to become the next prime minister.

"He has basically thrown this fantastic diplomat under the bus to serve his own personal interests," Duncan said.