The group has claimed that the top five price comparison websites promote tariffs from energy suppliers that pay them a £50 commission when a consumer switches, while cheaper deals are not promoted due to filters used in search options.
The Big Deal has written a letter to the five websites, in which it complains that they all use a search mechanism that ask a consumer if they want to switch "now" or "today". When a user clicks "yes", the search only shows up those companies which earn the site a commission.
Only if a consumer clicks "no" are they shown deals from other providers. The reason for this is that consumers are only able to switch suppliers "now" or "today" if there is an agreement in place between the price comparison site and the energy provider.
According to various reports, the price comparison sites have insisted that their services were transparent and conformed to regulations. They argued that the options on their websites were clear.
However, regulator Ofgem is reviewing its code of practice to better protect consumers by ensuring that websites display all tariffs.
The Big Deal has also written to complain to the Competition and Markets Authority, which is in the midst of an enquiry into the energy market.
This is not the first time that the price comparison sites have been accused of a lack of transparency. In January, the boss of Co-operative Energy accused them of misleading consumers over how much they were secretly charging in commission.