Debt as a percentage of gross domestic product (GDP) - a broad measure for the total economy - hit 65% in May, up from 61.3% last year.
April's borrowing figures were flattered by a one-off £28 billion lift from the value of assets transferred from the Royal Mail pension plan.
But excluding this one-off impact, total borrowing for the current financial year stands at £28.4 billion.
Earlier this month, the Chancellor and Governor of the Bank of England Sir Mervyn King unveiled a multibillion-pound lending scheme to stimulate economic growth - but ministers insisted the move was not a "Plan B".
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