The CMA has begun an investigation into the online secondary tickets market and whether buyers are being given the information that is required by law.
Consumers should be told who the seller is, any connection between the seller and the platform and the event organisers, any restrictions on the tickets that could cause the buyer to be denied access to the event, and where seats are located, the CMA said.
The four main secondary ticketing websites have previously committed to improve the information that they provide. A review earlier this year found that one was not fully complying with its obligations, and the CMA is working with the organisation to resolve this, it said.
However, the review also revealed "wider concerns about information provision and compliance with consumer protection across the sector" which have prompted the latest action, the CMA said.
The CMA will consider whether it believes the businesses selling tickets, and the platforms advertising them, are in breach of the law in relation to the information they provide. If so, it will take enforcement action, the CMA said.
The authority is also working with event organisers to make sure that any terms restricting resale of tickets are fair to consumers, it said.
Andrea Coscelli, chief executive of the CMA said: "We have heard concerns about a lack of transparency over who is buying up tickets from the primary market. We also think that it is essential that those consumers who buy tickets from the secondary market are made aware if there is a risk that they will be turned away at the door."
"We have therefore decided to open a sector-wide investigation to ensure that customers are made aware of important information that they are legally entitled to. If we find breaches of consumer law, we will take enforcement action," he said.
The four main secondary ticketing platform websites in the UK are GET ME IN!, Seatwave, StubHub and viagogo.