The pound rallied back up to 1.51 US dollars after the Bank of England decision not to change the rate of quantitative easing.
The pound had earlier dipped below 1.50 against the US dollar on fears that the Bank would restart the printing presses.
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Sir John rejected the PM's Brexit vision as "unreal and over-optimistic" as he accused negotiators of "souring" the atmosphere for talks.
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White House budget officials outlined the information during a telephone call with reporters given on condition of anonymity.