Around one in four adults have used online services such as MySingleFriend or Match.com, according to the study.
Around 40% of these people have discovered fake profiles and 20% - one in five - have been asked for money, consumer group Which? revealed.
It has warned users against sharing their personal details online.
"Be cautious of someone who quickly asks to talk on an outside email or messaging service, or who claims to be from the UK, but says they're travelling, living or working abroad and need money," Which? said.
It has advised anyone using internet dating services to:
- Avoid revealing details such as pets' names or their mother's maiden name which could be used to access financial information;
- Avoid including a surname, address, workplace or phone number in profiles;
- Report any request for money to the dating website.
The consumer group surveyed 2,052 adults about their online dating experiences in May.