Wedding venues must "play fair" over deposits and cancellation terms or risk enforcement action, the competition watchdog has warned.
The Competition and Markets Authority (CMA) has advised more than 100 wedding and event venues that large deposits and cancellation charges could breach consumer law.
The CMA said consumers who have to cancel or change their plans could face "considerable losses" because of potentially unfair terms after hiring venues at a cost that often runs to thousands of pounds.
Venues have been told that a deposit is simply to make a reservation and should be no more than a small percentage of the total price and advance payments should reflect the business's expenses and leave a reasonable amount to pay on completion.
It has also warned that customers must not lose large advance payments if they cancel, and venues should set sliding scales of charges to cover any losses.
CMA consumer senior director Nisha Arora said: "Planning a wedding or any large event can be stressful. Consumers are particularly vulnerable when they are focusing on preparing for a special event and have paid significant sums up-front.
"Businesses need to treat their customers fairly and should not require unjustifiable, non-refundable deposits or impose unreasonable cancellation charges, which could mean customers lose a significant amount of money if they change their mind about the venue or have to call off the event."
She added: "Unfair terms, even when a contract is signed, are not legally binding and we encourage any businesses which use advance payments and cancellation charges to review their terms to ensure they comply with consumer protection law.
"Many businesses in this sector comply with consumer protection law and engage in good business practices, but we urge others to raise their standards. Businesses that use unfair terms risk enforcement action."
Leon Livermore, Chartered Trading Standards Institute chief executive, said: "Getting married should be one of the happiest and most exciting days of a couple's life, but sadly it doesn't always go to plan.
"If a wedding has to be cancelled or plans have to be changed, couples could face losing out on considerable amounts of money after putting hefty deposits down to secure their dream venue or location.
"When you are trying to plan the perfect day, it is often easy to overlook certain contractual details during the planning stages but consumers need to be cautious before entering into a contract and always be aware of any costly cancellation charges or non-refundable deposits."