US president Barack Obama has suggested it is "hard to imagine" that it would be in Britain's interests to leave the European Union.
Mr Obama said the US valued the presence of its close ally at the top table in the EU and he did not believe it would be good for the 28-member bloc for Britain to leave.
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Thousands of people have linked hands on Westminster Bridge in a show of solidarity - one week after the terror attack in the capital.
PC Keith Palmer, 48, stabbed by Khalid Masood while on duty in the grounds of the Palace of Westminster last Wednesday.
Engineering firm CH2M won the £170 million contract but have pulled out of phase two of the project over alleged conflicts of interest.