Households ran down their savings to a record low in late 2016, according to official economic growth figures.
In its third estimate of growth for the final three months of 2016, the Office for National Statistics (ONS) confirmed the economy grew by 0.7%.
Economists said that was driven by a rise in consumer spending, helping to offset a fall in business investment.
But the figures show this spending was funded by dipping deeper into savings and running up debt.
The savings ratio fell to 3.3% in the final three months of last year, the lowest level since records began in 1963, the ONS said.
The ratio has been falling since mid-2015, as consumer spending has risen by more than households' disposable incomes have, it said.
John Hawksworth, chief economist at PwC, said: "On the surface, today's national accounts data reconfirmed the earlier picture that the UK economy remained relatively robust in the second half of 2016 following the Brexit vote.
"Beneath the surface though, some more worrying signs are evident."
Among them is that household net borrowing in the fourth quarter rose to over £11bn, the highest quarterly level since 1987, he said.