Warning letters have been sent to 35 discount websites telling them to review their practices, the consumer watchdog has disclosed.

The action follows the Office of Fair Trading’s investigation into Groupon - the discount voucher site with about one million subscribers - that found many of its practices potentially breached consumer regulations.

Groupon was given three months to overhaul its business and has now satisfied the regulator with the changes it has made.

However, the OFT has turned its attention to other sites after it found that consumers were often saving much less than was being claimed.

Some sites advertise discounts of up to 90 per cent on goods and services, with their daily deals used by thousands of consumers.

However, the OFT said many deals carry restrictions on when they can be claimed while others appear to be “time limited”, when in fact they are available for longer, pushing consumers into rash decisions.

The OFT also highlighted bait pricing, which tempts consumers with offers that cannot be fulfilled.

The watchdog said: “The OFT has informed the daily deal companies that the Groupon undertakings set a clear standard for the entire sector.

“The OFT is monitoring complaints about unfair commercial practices in this market and, where necessary, will take targeted national enforcement action against firms using practices that constitute serious breaches of the law.”

The OFT started investigating Groupon last July due to consumer complaints and a referral by the Advertising Standards Authority, which highlighted “serious concerns” about the company’s ability to stick to advertising codes.

Groupon uses “collective buying power” to offer discounted deals for customers by promising firms a minimum number of customers taking up the offer.