WhatsApp users have been warned about a "mum and dad scam" which has seen one woman conned out of £16,000.
Scammers have been posing as family members to convince their victims to transfer funds, before disappearing without a trace.
Police have now issued a warning over the tech fraud, saying they have seen an "increase in reports" of the scam, and urged people to "remind your friends and family to remain vigilant".
Which? has reported people being contacted by random phone numbers claiming to be a son, daughter or other family member or friend who suddenly has a "new number".
The scammer will then try and convince them to send money to various accounts, or pay for goods.
Some fraudsters will gain access to people's chat history to manipulate their victims and continue conversations in progress, cunningly manipulating the course of the conversation into a request for money.
Paula Boughton, from Paignton, Devon, fell victim to one such scam after she was contacted by someone claiming to be her daughter, Sam.
She told the BBC she believed she had been making payments on behalf of her daughter, after exchanging "personal and genuine" messages.
In total, she paid around £16,000 to a person she believed to be her daughter.
Her case is now being investigated by the City of London Police's National Fraud Intelligence Bureau.
She said: "I received a text message from what I presumed was my daughter, asking me to delete the old phone number as she'd been given a new number.
"It went on after a couple of lines of text to ask me if I would make a transaction for her, which I agreed to, if she sent me the sort code, the payee's details, and the account number.
"I presumed it was my daughter, and I thought, well, because of the situation, I was able to do that, and I feel that I've been made a fool of."
Sam said she was "furious" when she learned what had happened and added: "Why on Earth, when I know she's savvy with things and technology and you know, knowing there's a lot of fraud out there and fake scams, why would she pay that much money?"
How can you avoid being scammed?
If you get a close friend or family member asking for money, give them a phone call on their original phone number - just to double-check they are telling the truth.
Don't ever give out security codes for any accounts to anyone.
Alternatively, asking them to send a voice note to verify who they are.
If in doubt, then don't send the money.
WhatsApp's policy manager, Kathryn Harnett said:
She said: "WhatsApp protects our users’ personal messages with end-to-end encryption, but we want to remind people that we all have a role to play in keeping our accounts safe by remaining vigilant to the threat of scammers.
"We advise all users never to share their six-digit PIN code with others, not even friends or family, and recommend that all users set up two-step verification for added security.
"And if you receive a suspicious message (even if you think you know who it’s from), calling or requesting a voice note is the fastest and simplest way to check someone is who they say they are.
"A friend in need is a friend worth calling.”