Shadow Foreign Secretary Douglas Alexander said:
On the same day that top British business leaders fired a warning shot across the bows of the Prime Minister, a senior White House official has also signalled concerns about the possibility of Britain leaving the EU.
There is today a real risk of Britain sleepwalking towards exit because of a Prime Minister motivated more by the need for party unity than by the interests of the country.
More top news
The Westminster terror attack dominates the front pages of Thursday's newspapers.
A timeline of exactly what happened.
Tobias Ellwood, MP for Bournemouth East and Foreign Office Minister, attempted to save the life of the stabbed officer in Westminster.