Vodafone has added a sneaky £1-a-month charge to more than one million customers' mobile phone contracts, we can reveal.

The new fee — which will net the telecoms giant millions of pounds a year — has been added to every new monthly contract taken out since February.

Vodafone says the service offers customers extra protection against any harmful websites users may visit while surfing the internet.

The so-called 'Secure Net' software is included on monthly contracts and is advertised as a 'free three-month trial' in Vodafone's promotional material.

Grab: Vodafone is charging customers £1-a-month for software to protect against any harmful websites users may visit while surfing the internet

But customers claim they are not told they have been opted in and will be charged £12 a year when the free trial ends. It is impossible to remove the extra software at the time of purchase and it cannot be cancelled until at least 24 hours later.

Money Mail has discovered that the cost is not included in its pricing small print if you take out the contract online. Even if you sign up in store, it is not always mentioned by staff and is glossed over on the agreement you sign.

Instead, customers are notified by text after signing their contract and it is then up to the customer to remember to cancel the add-on if they want to avoid the fee.

Last night the telecoms watchdog Ofcom said it would look into whether Vodafone was making the fees clear enough. And Vodafone responded by saying it would give staff extra training and review the wording on its website.

Martyn James, of complaints website Resolver, says: 'Automatically opting people in to services that cost money is an unethical and misleading practice. Industries from car hire companies to airlines have all had their knuckles rapped over automatically opting people in to things such as insurance.

'And given that none of the Vodafone customers we've spoken to were aware of this charge, it's likely that a large number of people are going to be very angry that they've been hit with such a sneaky charge.'

Money Mail receives a regular stream of complaints from customers of all mobile phone companies who have been signed up to monthly subscriptions without realising. Many then struggle to cancel and get a refund.

Vodafone insists staff are supposed to tell in-store customers about the fee when they take out a contract.

But when a reporter asked a London branch for a pay monthly deal it was not mentioned. When we raised the add-on after signing the contract, a sales assistant said they were not instructed to tell people about the charge.

The service is mentioned only in passing on the agreement in a line which reads: 'The monthly charge for your extras: 4G £0.00, Vodafone Secure Net £1.00, Vodafone Global Roaming £0.00.'

Vodafone, which has 18 million UK customers, said around one million customers now have the extra Secure Net software and that 70 per cent 'decide to retain Secure Net after the free trial'

If you buy your contract via the website, the £1 charge is not mentioned in the main pricing details or in its in-depth 'full list of prices'. Instead, when you click on the deal you want, you are told your offer includes a 'three-month free trial of Secure Net'.

To learn about the add-on charge you would then have to scroll down and seek out more details of Secure Net by clicking on a link about the software.

Vodafone says it sends a text message to customers one day after taking out the contract, offering the opportunity to opt out, and a further four during the trial. But angry customers say they were not aware of these 'hidden' charges.

One wrote on a forum: 'This is pretty cynical on the part of Vodafone. By all means enhance security, but if it's not going to be free after a trial they opted you in to, you should not need to have to cancel yourself.' Others have expressed confusion after the charges appeared on their bill.

Vodafone customers say they are not warned that they will be charged when the trial ends

One Twitter user wrote: 'What's this £1.00 Vodafone Secure Net charge showing on my bill?'

Financial adviser Al Rush, of Rutland, tweeted: 'Anyone else mysteriously being charged for Secure Net without giving Vodafone UK your express permission?'

Another customer named Dan, an analyst from Sheffield, tweeted: 'What is Vodafone Secure Net? Why do I have it without asking? How do I get rid of it?'

When Money Mail contacted Vodafone's customer services department about the unwanted add-on, staff revealed the firm immediately refunds customers who claim they were not told about the monthly fee.

He said: 'I am sorry it was not informed to you. If it gets charged without knowledge of the customer, we remove the charges.'

He insisted customers were usually notified by text about the charges when their three-month trial expired and it was 'very rare' to neither be told about it in store nor alerted via text.

Vodafone, which has 18 million UK customers, said around one million customers now have the extra Secure Net software and that 70 per cent 'decide to retain Secure Net after the free trial'.

Secure Net offers extra protection against viruses, malware and phishing sites, Vodafone says, to stop customers' phones becoming infected. 'If any online threats are detected they'll be stopped before they get anywhere near your device — and Secure Net will send you a text message to let you know,' it says.

A Vodafone spokesman says: 'Secure Net is listed where included in a plan and a [web] link provided for additional information. We will review this to see if there are ways we can improve the customer experience.

'As with any new service, we will continue to look to make improvements to the sales process and to the service itself.'

Vodafone says its 'Secure Net' service offers customers protection against harmful websites

He added: 'Stores have been trained to inform customers of the trial and the subsequent charge.

'We will provide them with a refresher to ensure that this happens in all cases.'

Vodafone says that employees in branches are expected to explain that the three-month trial is included. Rival firms O2, Three and EE said they did not apply any automatic charges for additional security features or software.

A spokesman for Ofcom says: 'We expect customers to be treated fairly, and any additional charges to be made clear at the point of sale — whether in store or online. We're discussing this process with Vodafone.

'If a customer was not made aware of the terms of the Secure Net trial at point of sale, or they did not receive the information in their Welcome Pack and text, they should complain directly to Vodafone.'

l.eccles@dailymail.co.uk