The CMA has launched a probe into unfair sales tactics including misleading offers, extra charges and drip pricing, which pressures consumers into buying quickly or spending more than they intended.
Following results of an investigation that began in April, the CMA has now sent out 100 advisory letters after reviewing more than 400 businesses.
Holidays, including package travel, topped the list of the 14 types of businesses of concern to the CMA, with 19 advisories sent. This was followed by live events with 15 letters and parking with 14.
However, no travel businesses are among the eight firms into which the CMA has formally launched investigations.
