When my 87-year-old mother, Patricia Collinson, was given an Alexa speaker by my sister, she was delighted to find she could ask it to say the Hail Mary. Every morning for a week the devout Catholic asked Alexa to recite the prayer.

What she was less delighted to learn was that she had unwittingly ordered a premium subscription payable through Amazon to a private company called Catholic Prayers.

Patricia, a retired district nurse in Hastings, does not own a computer, and does not know how to use one. She had signed up by voice command, without being presented with the kind of outline or terms and conditions that now comes as standard when you pay for things online.

Her experience throws a spotlight on a relatively new phenomenon, Alexa “skills”. Launched in the UK in 2016, these are the voice service’s version of apps. There are 45,000 in the UK, which range from security offerings (such as enabling your Alexa to hear breaking glass or a smoke alarm) through to recipe ideas and even “send a hug” services.

Although they are usually free to order verbally over Amazon’s Alexa, many also encourage in-app purchases – which can be made simply by saying “yes”.

Patricia says that at no point did she understand she was making a purchase or entering into a subscription.

“I got into the habit most mornings of coming downstairs, sitting in my recliner and saying: ‘Good morning, Alexa. Can you say the Hail Mary please,’” she says.

“It never asked for money. It never said it was charging me. It was completely news to me.”

The Alexa was set up by my sister, Catherine, and is attached to her Amazon account. She spotted an unusual email from the retailer, which said: “Order confirmation. Your payment has been processed and your subscription term has started.”

Fortunately the subscription was only for £1.99 a month, and there was a free seven-day cancellation period, which Catherine caught just in time. But my sister is still befuddled as to how my mother could have entered into the contract, and how, had she not noticed the email, my mother would now be paying.

“Thank goodness she didn’t ask Alexa to say the Rosary,” my sister joked, referring to the set of prayers that includes 53 Hail Marys.

But there is a serious point here. Digital voice assistants, such as those from Amazon and Google, are now in millions of UK homes. Vulnerable consumers including children and the non-computer-savvy can inadvertently enter into premium subscriptions simply by saying yes.

In a statement, Amazon said: “For purchases by voice, customers can buy content by saying yes to a product offer message, generated when a customer requests the product directly or when the customer responds positively to a proactive suggestion within the applicable skill.”

It added: “Following a voice purchase, customers receive an order confirmation email from Amazon. With the Catholic prayers skill, customers are offered a free seven-day trial after which time they are asked if they would like to continue their subscription for a fee.”

It was fortunate my sister spotted the email from Amazon, or the payment would be going through each month.

Guardian Money contacted the developer behind Catholic Prayers, Nicholas Azzarello, who lives in the US. He said he was very sorry for the trouble he had put my mother through, adding that “she may not have realised what Alexa was asking her when she said yes”.

He added: “Maybe your mother said yes and then Alexa explained to her how much the premium subscription would cost a month after the seven-day trial. Then Alexa asked if she is sure she would like to sign up and your mother may have said yes to that as well, even though she may not have fully understood what Alexa was talking about.”

Azzarello said Catholic Prayers had about 10,000 users a month, but was not in any way affiliated with the Catholic church itself.

Amazon said that for skills aimed at children approval was required for each voice purchase and parents could disable voice purchasing. “Developers are required to identify if their skills are directed to children when submitting the Skill to Amazon,” it said.

When Guardian Money asked Google’s Assistant for the Hail Mary, it was provided without payment, or any attempt to seek payment. We were unable to find any premium service charging for similar content.

But for many Alexa developers, the skills service has enabled them to earn large sums.

On its commercial pages, Amazon encourages IT developers to “Earn Money with Alexa Developer Rewards”. It boasts of how one developer in the US earned $25,000 in six months from “good night messages” over Alexa that he had produced just “for fun”.

How to disable voice purchasing through Alexa

There are several ways to prevent accidental purchases of skills, goods or services via voice through Alexa, all accessed via the Alexa app on your Android phone, iPhone or tablet.

Open the app and navigate to: More > Settings > Account Settings > Voice Purchasing.

Here you will find the ability to disable voice purchasing entirely or set a pin that must be spoken to authorise transactions. You can also lock down purchases to only certain profiles by recognising an individual’s voice ID such as yours but not your children’s.

In addition, voice purchasing within kids’ skills can be turned off entirely or set to “require purchase approval”, which sends a text message and/or email to the registered account holder to sign off. Sam Gibbs