UK consumers are being ripped off to the tune of £71billion a year by firms selling poor quality products, mail order goods that never arrive and services that are never delivered. The shocking cost to the British public also includes subscriptions that cannot be cancelled, so that customers continue to be charged, and late-running or cancelled trains, leaving season ticket holders out of pocket.

MPs are investigating after Citizens Advice warned unwary customers face a "wild west". The scale of the problem was revealed by the Competition and Markets Authority (CMA), a government body. A study found that over the course of 12 months, the total cost of "consumer detriment" in the UK came to £71.2billion.

Customers reported buying clothes online that didn't fit because the wrong size had been sent, food that triggered an allergic reaction after ingredients were listed wrongly, and products that turned out to have missing parts once the box was opened.

Some firms refused to provide replacements or refunds for faulty products.

Other issues included problems with the internet, mobile phone and subscription television services.

A report by the National Centre for Social Research, commissioned by the CMA, said: "This translates to approximately 38.5million consumers experiencing at least one problem with a product they bought or used during that period, which caused them stress, cost them money, or took up their time."

The House of Commons Business Committee has launched an inquiry. Chairman Liam Byrne said consumers were being hit by a combination of shoddy goods and high inflation.

He said: "One of the things that shocked us is that the hit to consumers is not just higher prices - there is now this wide range of harms that cost Britain's consumers £71billion a year.

"That is everything from poor quality goods that you cannot get a refund for, subscriptions that you cannot cancel, deliveries that do not turn up, stuff that is just unsafe, and services that you pay for but never materialise.

"That £71billion is thousands of pounds for every house in the country, on top of these increases in prices."

Anne Pardoe, head of policy at Citizens Advice, said thousands of people called the charity's free helpline to ask for advice on poor products or services.

She said: "The two areas where we receive the most calls are used cars and home improvements. Last year, we got 57,000 calls from people about the used car market, and 38,000 around home improvements.

"We have also previously highlighted issues around markets that are smaller but really important to the people who use them - for example, disability aids.

"But what really unifies these markets is the fact that they are very lightly regulated, and you do not have a sector regulator.

"Trading standards do a good job with the resources they have, but it feels like a bit of a wild west out there."

Trading standards departments had experienced budget cuts of 39% in real terms in recent years, said John Herriman, chief executive of the Chartered Trading Standards Institute.

He warned: "So you have a depletion in capacity at the local level."