The Financial Conduct Authority (FCA) is warning people to beware of people posing as investment advisers and offering to help them set up new schemes via online meeting platforms.
They ask their victim to share the screen and enable remote access - which hands over control of their device and, potentially their bank account.
About 2,100 cases have been reported to the FCA since July 2020. More than £25m was stolen in the 15 months from January 2021.
The FCA said the number of screen-sharing scams it had seen had risen 86% to 683 in the six months from July to December 2021 compared with 367 for the same period the year before.
Screen-sharing has become a familiar part of work life, as people use popular online meeting programmes in their jobs.
Remote access software is a legitimate tool for services like IT support to troubleshoot problems without being in the room.
But scammers are increasingly hijacking this familiarity to lure victims into granting access to more than just a picture of their screen.
They are then persuaded to grant the fraudsters control of their computer, by either expanding permissions or downloading remote access software, giving them direct access to online bank accounts. It also means they can install their own malware giving them full access at any time.
The criminals do this under the guise of being helpful - offering to set up a new investment scheme and monitor it.
One scam victim, who wished to remain anonymous, says she lost thousands of pounds to scammers after trying to find new investments to grow her savings.
She was called by a seemingly legitimate investment company in spring 2021 and told she could see a sizeable return on an initial £250 investment by downloading remote access software and letting the firm do the rest.
"They didn't say [the software] was anything other than an investment tool, and so I had no idea what it would actually do," she said.
Over the following six months, the woman was encouraged to make more investments - saying graphs and information from scammers saw her "blinded by science", and led her to believe her investments were paying off.
But by that time, the scammers had drained her pension fund by £48,000 and taken out a further £40,000 of loans in her name.
"I didn't know what was happening until it was all gone," she says.
Mark Steward, executive director of enforcement at the FCA, said:
"Investment scams can happen over many months, but sharing your screen without making the proper checks can change everything in an instant."
If scammers gain control of your computer, it gives them "access to your sensitive banking and investment information, the freedom to browse at their leisure, and the ability to take whatever details they want"
The FCA's ScamSmart website has further advice.
Rocio Concha, director of policy and advocacy at consumer watchdog Which?, said:
"Screen-sharing scams are often incredibly sophisticated and, as the FCA rightly recognises, even the most experienced investors can be taken in by these fraudsters.
"If you have shared your screen with a scammer, try to take back control of your device by using the disconnect button, enabling you to end the session.
"As a precaution, you can turn off wi-fi at the router or unplug the network cable to fully disconnect from any external connection."
Reporting
Report all Fraud and Cybercrime to Action Fraud by calling 0300 123 2040 or online. Forward suspicious emails to report@phishing.gov.uk. Report SMS scams by forwarding the original message to 7726 (spells SPAM on the keypad).