Figures from the Office of National Statistics show that UK public sector borrowing totalled £8bn last month, the first July deficit for 13 years.
Public sector finances usually post a surplus in July because the month includes a quarterly corporation tax bill. Last year the surplus in July was £5.2bn.
Liberal Democrat Shadow Chancellor, Vince Cable commented:
"As the country fights its way through recession we are seeing a collapse in tax revenues.
"We are now heading for a level of deficit this year even higher than the Chancellor's original predictions.
"What is particularly concerning is that the Government's hopes for the recovery of the public finances are based on extremely optimistic growth forecasts. Without this growth we will be heading for even higher levels of debt."