“Romance fraud” cost its victims more than £50m last year, according to official figures.
It happens when people are tricked into thinking they have found an ideal partner on a dating app, only to be scammed out of money once criminals have gained their trust.
In some cases, fraudsters gain enough information to steal their victims’ identities.
According to Action Fraud the average loss per victim last year was £11,145 – an increase of more than a quarter on 2017.
Almost two-thirds of victims were female, and the average age of those scammed was 50. On average, women lost twice as much as men.
More than 4,500 reports of romance fraud were made to Action Fraud, which is run by City of London Police and the National Fraud Intelligence Bureau.
More than 40% of victims said being a victim of it had affected their health and financial wellbeing.
Techniques used by fraudsters include pretending to be someone they are not, using fake photos, being reluctant to meet in person and inventing sob stories to explain why they need large sums of cash.
In one case, a woman lost about £10,000 to a man claiming to be in the army, who said he needed money to get a box of belongings returned to the UK.
After he had gained her trust by talking about getting married and buying a house together, she used a loan and her private pension to cover the money she sent.
She eventually became suspicious and contacted the Foreign Office, which confirmed it was a scam.
Katy Worobec, managing director of economic crime at UK Finance, said: “We are urging customers to be vigilant against romance scams and not let a fraudster fool you this Valentine’s.
“Banks are always looking out for any suspicious transactions, but we need customers to be on guard against suspicious approaches too.
“Always be wary of any requests for money from someone you’ve never met in person.”