UK phone companies will make it harder for fraudsters to use their networks under Ofcom plans after almost 45 million people were targeted by scam calls and texts last summer.

All telephone networks involved in transmitting calls will be expected to block numbers that are clearly “spoofed” to better protect millions of people.

Spoofed numbers include calls originating from abroad that do not have a valid caller ID, using a number that does not meet the UK’s 10 or 11-digit format, and calls appearing to be from numbers that are already on Ofcom’s Do Not Originate list.

The regulator warned that nearly a million of the almost 45 million people who were targeted by scam calls and texts last summer followed the scammers’ instructions, risking financial loss and emotional distress.

Ofcom already works with phone companies to help them block calls that imitate – or spoof – the phone numbers of legitimate organisations, such as banks and Government departments.

But fraudsters were able to quickly adapt to changing circumstances and technology. During the pandemic, for example, criminals had been texting fraudulent vaccination links and impersonating delivery companies.

The new guidance on blocking calls from abroad that falsely use a UK number was based on an initiative developed by industry, which some providers have already implemented voluntarily.

TalkTalk said it had seen a 65% reduction in complaints about scam calls since it introduced this measure.

The regulator will also make it harder for scammers to access valid phone numbers by requiring phone companies to run “know your customer” checks on business customers, by verifying them from details on Companies House, fraud risk databases and the FCA’s Financial Services Register to test whether a “high risk of misuse” exists.

For calls originating in the UK, this would involve the network from which the call is being made “authenticating” the caller’s ID information before connecting them.

Huw Saunders, Ofcom’s director of network infrastructure and resilience, said: “The threat posed by scammers has grown significantly in recent years, and the sophisticated tactics used by these criminals can have devastating consequences for victims.

“We’re taking action so phone companies have stronger systems in place to disrupt scams. While there is no silver bullet that will end the scourge of scam calls completely, we’re working with industry on how we can use technology to make it as difficult as possible to reach people.”

Ernest Doku, telecoms spokesman at Uswitch.com, said: “For millions of Britons, scam calls and texts have worsened in the last two years. We’ve relied on phones to keep us connected to family, friends and work, yet our reliance has also left us vulnerable to scammers seeking to take advantage.

“Ofcom must ensure that any efforts to stop the scourge of nuisance calls also focus on landline phones, as our own research found that scammers now represent more than a quarter of all calls to our landlines. The volume of these scam calls for landline customers now means that one in five customers actively avoid answering their phone in case it’s a nuisance call.”

Rocio Concha, Which? director of policy and advocacy, said: “Consumers are being bombarded every day with scams, and exploiting weaknesses in the telecoms network has been one way that fraudsters have tricked victims out of significant sums of money, so it’s good to see the regulator taking action to crack down on this type of fraud.

“Companies should be doing more checks on business customers, and as fraudsters are constantly evolving their tactics, it’s important that Ofcom and industry leaders continue to work together on proactive network level solutions that prevent fraudulent activity.”