Tougher rules to get millions of people out from being trapped under billions in long-term credit card debt have been proposed by the financial watchdog.

The Financial Conduct Authority (FCA) will require credit card firms, such as Visa and MasterCard, to prompt around 3.3 million customers who have been in "persistent debt" for 18 months, to make faster payments.

The watchdog describes customers in persistent debt as those who spend more on paying off interest and other charges than the underlying debt.

It adds that within this larger total, 1.8 million customers have been in persistent debt for two consecutive periods of 18 months, according to its recent study of the UK credit card market.

FCA chief Executive Andrew Bailey said: "Persistent debt can be very expensive – costing customers on average around £2.50 for every £1 repaid – and can obscure underlying financial problems. Because these customers remain profitable, firms have few incentives to intervene."

The FCA said if customers are still under persistent debt 18 months after first being contacted by their credit firm, the companies must then propose a repayment plan.

Customers who do not respond, or refuse to take up a repayment plan, will have their cards suspended.

The watchdog added that to help customers weighed down by debt firms must take such steps as "reducing, waiving or cancelling any interest or charges". The FCA said it would expect firms to suspend a customer's card during this period.

The measures will mean customers will save between £3bn ($3.8bn) and £13bn ($16.2bn) in repayments by 2030.

Lending by banks to individuals grew by £4.9bn ($6.1bn) during February, according to Bank of England statistics published last week, slightly above the average of £4.8bn ($6bn) over the past six months.

Consumer spending has been the engine behind the economy's growth since the Brexit referendum last June, helping defy economists' expectations of a slowdown, although there are growing concerns over how long this can last and inflation rises, pushing up the cost of living.