Asked whether paying cash in hand was "morally wrong", Labour leader Ed Miliband said ministers should be focusing on large-scale tax avoidance.
Speaking on a visit to Paris, Mr Miliband said: "What I say is that the job of government is to pass the right laws to clamp down on tax avoidance - that's the most important thing of all.
"What I will be saying to the Government is that they should be clamping down on the large-scale tax avoidance which has been revealed in the past few days and I think that's what people want to see from the Government."
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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.