David Cameron said Britain has spent over £300 million and "saved tens of thousands of lives" in Syria.
The Prime Minister said: "I've been to Jordan and seen one of the refugee camps myself and I know how much money and how much effort is going in.
"Britain, as ever, is a world leader at helping those who need help, and the people of Syria are right up at the front of that right now."
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It reportedly threatened attacks on planes "flying from Heathrow to the US" across the July 4 US Independence Day holiday weekend.
Accompanied by the Duke of Edinburgh, the Queen will attend the opening ceremony of the fifth session in the Holyrood chamber.
Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.