Government borrowing surged 9% in the four months to July, despite public finances running their first July surplus in 15 years.
The Office for National Statistics (ONS) said public sector net borrowing, excluding public sector banks, increased by £1.9 billion since the start of the current financial year, reaching £22.8 billion.
That is despite an increase in tax receipts last month which helped deliver a small surplus of £200 million.
It marked the first July surplus since 2002, according to the ONS, and was driven by an £800 million year-on-year increase in receipts from self-assessed income tax to £8.0 billion, the highest level on record.
Britain's finances tend to enjoy a strong July as self-employed people pay their income tax and businesses settle their corporation tax bills.
But the jump in self-assessment tax receipts last month was down to "unusually low" comparative figures in July 2016, Samuel Tombs, chief UK economist at Pantheon Macroeconomics, explained.
He said that last year's deadline for payments fell on a weekend and instead bolstered August figures.
"The first July surplus since 2002 is not a signal that the economy is in rude health," Mr Tombs said.
Public sector net debt excluding public sector banks has climbed £143.9 billion year-on-year, bringing the UK total to £1.76 trillion, equivalent to 87.5% of gross domestic product (GDP).
Sterling was 0.5% lower versus the US dollar at 1.283 in the wake of the data's release, but was flat against the euro at 1.091.
The ONS also revealed that the Government delivered a smaller-than-expected deficit in the previous financial year, with borrowing excluding public sector banking falling by £27 billion to £45.1 billion in the 12 months to March 2017.
It undershot the Office for Budget Responsibility (OBR) forecast of £51.7 billion, and was the lowest net borrowing figure since the financial year ending March 2008.
However, the OBR is expecting a £13.2 billion increase in Government borrowing this year to £58.3 billion.
Public finances are likely to be hampered by a struggling UK economy and higher interest debt payments over the coming months, but Howard Archer, chief economic advisor for the EY Item Club, said Chancellor Philip Hammond is likely to have some "wiggle room" by the time he delivers the autumn budget.
"Any November wiggle room for the Chancellor will be welcome given that increased public dissatisfaction with austerity and on the public sector pay cap has put pressure on the Government to recalibrate fiscal policy," he said.
"Even so, the suspicion is that Chancellor Philip Hammond is much more minded to make limited tweaks to the fiscal approach in November's budget rather than radical policy changes."