An "extremely convincing" scam which redirects people to the official Post Office website may put many people at risk of becoming victims, Which? has warned.
The Echo reported that the consumer group said it had seen variations of texts claiming a parcel delivery had failed and asking recipients to click a link to book or reschedule a delivery.
Clicking on links may take people to convincing cloned online sites, Which? said, and they are asked to fill in their personal details, putting them at risk of identity fraud.
They may then be asked to pay a charge for redelivery. Users may be told their redelivery request has been "processed successfully", making the scam appear even more convincing.
However, the Post Office does not send out texts about parcels or mail and it said it takes immediate action to have fake web pages taken down.
Adam French, Which? consumer rights editor, said: "If people input their details and pay for a new delivery date, the scammers redirect them to the official Post Office website - making this fake even more plausible.
"Consumers should be on high alert for scams and, if in any doubt, should verify the text directly with the company before giving any personal information.
"If you've entered any bank details, contact your bank immediately to ensure the scammer cannot take any more money from your account, and ask to be reimbursed. You can also report any attempted scam to Action Fraud."
A Post Office spokesperson said: "People are being tricked by fake texts claiming to be from Post Office, but the Post Office will never send texts about parcels or mail.
"This is because Post Office does not deliver parcels or letters. We're grateful to Which? for alerting us to this fake website which we were already aware of and had taken action to get it blocked.
"Cyber criminals are frequently trying to get people to click on links in texts regarding undelivered shipping fees and undelivered items and directing them to fake websites purporting to be an official Post Office website.
"We are working hard to raise awareness about these fake texts in our name to help stop people being conned.
"As soon as we are aware that a fake Post Office web page is up and running we take immediate action to have the site taken down.
"This week we have launched a social media campaign aimed at those aged 50-plus to warn them of scams that may use the Post Office name and what to do if they have any suspicions."
People can report scam texts by forwarding them to the network provider on 7726 and it is worth alerting the genuine companies that scammers are imitating, Which? said.
The Post Office spokesperson added: "Post Office use digital monitoring systems that alert us to any website set up with our name in the URL.
"Once we are alerted (within hours to 24 hours of a URL being registered) we monitor the URL for any web page that is set up, specifically using our brand.
"We are only able to take action once a website is set up and displays our branding and trademark."
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