"Callous" crooks pretending to be water board inspectors are calling on elderly people's homes asking to take water samples to test for Covid-19.
Once inside, they look for money and valuables before fleeing the scene.
Two men in Warrington wore hi-viz jackets and used a white van with fake United Utilities branding on it, with another conman in Accrington working alone.
One pensioner has already had money stolen and United Utilities is now urging customers to be extra vigilant and wary of strangers.
Louise Beardmore, customer service and people director of United Utilities, said: "We're appalled that people could be so cruel when we need to be pulling together.
"No-one from UU will call on you unless you ask them to. If in doubt, just shut the door. Never let anyone into your home unless you are confident they are who they say they are.
"If you haven't booked a visit, then people who claim to be from us are bogus, even if they are wearing a United Utilities uniform or if there is one of our vans in the street."
United Utilities has advised that you should attach your door chain if you have one and ask for identification.
Ask the caller to hold their ID card up to the window or put it through your letterbox so you can check it without opening your door.
If you're not sure, tell the person to wait outside and call United Utilities on 0345 672 3723 to check their identity before letting them into your home.
United Utilities has also encouraged customers to keep an eye on vulnerable neighbours.
Insp Ruth Atherton, of Warrington Local Policing Unit, said: "Criminals are experts at pretending to be people they are not. I urge residents to be vigilant and to not let anyone into your home who calls unexpectedly.
"It's okay to reject or refuse requests from people you don't know. Only criminals will have an issue with being told that you won't let them into your home.
"I also urge residents to keep an eye on neighbouring properties during the lockdown in case bogus callers target vulnerable people living there."
United Utilities offers a password scheme for pre-planned appointments to give you added security.
For more information visit http://www.unitedutilities.com/bogus-caller.aspxPeople with information about the suspicious activity can call police on 101.
Alternatively, independent charity Crimestoppers can be contacted on 0800 555 111 or via their website https://crimestoppers-uk.org/