Britain stood isolated in the eyes of the world yesterday after “special relationship” ally Washington announced it will join an international conference on nuclear weapons that the Tory government is poised to boycott.

The US State Department decision to attend the Humanitarian Impact of Nuclear Weapons summit in Vienna is a major embarrassment for the Foreign Office just days after Undersecretary of State Tobias Ellwood suggested in the Commons that “the goals of the conference are unclear” and no permanent UN security council member would go.

He was left red-faced yesterday as the US stated its belief that the event — set to include over 150 countries — would see “real prospects for constructive engagement with conference participants.”