Expedia, Booking.com, Agoda, Hotels.com, ebookers and trivago have been investigated over high-pressure selling tactics and misleading discount claims, the competition watchdog says.
The Competition and Markets Authority (CMA) was concerned the sites were making rooms seem more popular than they were.
The sites will now say if commissions they receive affect the results.
They also agreed to be clearer with discount claims and hidden charges.
The CMA began its investigation in June 2018 but didn’t name the comparison sites it was investigating.
“The CMA has taken enforcement action to bring to an end misleading sales tactics, hidden charges and other practices in the online hotel booking market,” said CMA Chairman Andrew Tyrie.
“These have been wholly unacceptable.”