The Prime Minister launched into a brief argument for Scotland remaining as part of the UK after the Scottish referendum in 2014.
He said he believes the case for union can win the "arguments of the heart" as well as the "arguments of the head".
He said he is proud of what the UK has achieved together, such "defeating fascism" and the NHS and BBC.
But he also drew attention to the more than 12,000 people in Scotland employed by the defence industries, which he said were backed by the whole of the UK.
More top news
In an embarrassing setback for the president, Republicans failed to reach the number of votes to pass his new healthcare act into law.
It will feel pleasantly warm in the sunshine, with temperatures reaching 12 or 13 Celsius (54 to 55F).
Some of Britain's biggest stars of the stage and screen will be honoured at Buckingham Palace later today.