U.S. government borrowing has hit “unprecedented” levels that could trigger a damaging market reaction if left unchecked, the head of Congress’ independent fiscal watchdog warned in an interview with the Financial Times.

Federal debt relative to gross domestic product is likely to rise above World War II levels — when it stood at 116% of GDP — by 2029, Phillip Swagel, director of the Congressional Budget Office cautioned.

Ignoring the debt issue runs the risk of a market shock similar to the 2022 selloff in the U.K., when plans for sweeping tax cuts led to a run on the pound and forced then-Prime Minister Liz Truss to resign, he said.

While the U.S. is “not there yet,” government debt held by the public — which stood at $26.2 trillion, or about 97% of GDP, at the end of last year — is on an “unprecedented” trajectory, Swagel said. Bloomberg Economics has projected that in 88% of a million simulations, the country’s debt is “on an unsustainable path.”

Skyrocketing debt combined with high interest rates mean the country might not be able to afford crucial borrowing in the future, with the U.S. set to pay $1 trillion to creditors in 2026, the FT said. It could also, the CBO warned in a report, “erode confidence in the U.S. dollar as the dominant international reserve currency.”