The Bank of England has forced banks to find a further £11.4bn in the next 18 months to beef up their finances against the risk of bad loans.
Banks will have to set aside £5.7bn in the next six months in case future economic shocks mean some borrowers cannot keep up their repayments.
A further £5.7bn will have to be found by the end of next year.
The Bank's Financial Policy Committee (FPC) suggested lenders had become complacent about their lending.
"Lenders may be placing undue weight on the recent performance of loans in benign conditions," the FPC said.
The committee has also taken action to stop banks getting around key tests which are designed to stop them lending too much to consumers.
The FPC's assessment is that the risks facing the financial system remain at a normal level for now.
But there are "pockets of risk that warrant vigilance" it said, in the Bank's half-yearly Financial Stability Report.
Lenders, the committee said, are relying too heavily on borrowers keeping up payments as well as they have recently, and banks and other lenders have started lending to people with weaker credit records.
The FPC highlighted rapidly growing consumer borrowing via credit cards, personal loans and, notably, car finance.
Collectively known as consumer credit, these forms of borrowing have grown by more than 10% in the past year, far outstripping the growth of incomes.
While the amount of borrowing for consumer credit is just a seventh of the size of mortgage lending, the amount lenders have to write off because it is not likely to be repaid is ten times greater than for defaulting mortgage borrowers.
In a news conference, the Bank Governor Mark Carney explained that the Bank was worried about those households who are heavily in debt.
But their borrowing, he said, had not in fact increased the threat to the general resilience of banks.
"We are reinforcing some of the protection [for banks]," he explained, by telling banks to add to their financial cushions.
He declined to blame people for borrowing more, and said that most personal borrowing decisions were reasonable.
However, he advised: "Borrowers should consider adverse scenarios as well as positive scenarios."
Stronger financial system
The Bank is bringing forward by six months a so-called "stress test" in respect of consumer credit, whereby lenders have to test their ability to withstand losses on loans that go bad and are not repaid.
It is also blocking lenders from getting around affordability tests for lenders designed to stop them over-lending on mortgages.
Banks and building societies are currently allowed to lend a maximum of 15% of their mortgages to homebuyers who take especially large loans of more than four and a half times their income.
The lenders have to scrutinise the borrowers to ensure they could still afford their repayments if the Bank of England raised its official base rate by three percentage points.
But some lenders have been assuming they would not in fact pass on all of that increase in higher standard variable rates, thus allowing them to lend slightly more.
Mr Carney said these lenders were not "gaming the system" but instead appeared to have forgotten some of the lessons of the recent past.
Despite these concerns, Mr Carney stressed that the UK financial system was far stronger than at the time of the great banking crash in 2008-09.
He said that since then, UK households had reduced their levels of debt and that it was only in the past 18 months or so that personal lending and borrowing had accelerated again.
"The resilience of the UK financial system has strengthened since the financial crisis," Mr Carney said.