If you shop at Tesco over the next month, you may be asked if you want to round up your bill to the nearest 10p to raise money for the charities Diabetes UK and the British Heart Foundation.

Tesco is supporting two brilliant causes with this initiative, and it's obviously fantastic to give to charity if you can afford it. Yet while many have welcomed the scheme, we've also heard from some who claim donations have been added without consent – including a support worker for disabled adults and a single mum who says "every penny counts".

The supermarket's urging customers to add a few pennies to their bills as part of a wider charity drive which has also seen thousands of Tesco staff set off this week on a sponsored month-long trek between Land's End and John O'Groats.

Rounding up your bill is supposed to be entirely voluntary – you'll be asked to choose whether or not to donate at self-service machines, and should only have your bill rounded up at staffed checkouts if you explicitly offer to donate.

For more on how to max your donations to charity, see our Top Charity Giving Sites guide, while for full help on how to keep the cost of your grocery shopping down, see our 36 Supermarket Shopping Tips.

What is the scheme and how do I donate?

Anyone shopping at Tesco until Thursday 5 October will be encouraged to take part in the scheme. If your bill isn't a multiple of 10p, you'll have the option of rounding it up to the nearest 10p – so for example, if your shop comes to £1.86, you can round it up to £1.90, with the extra 4p going to charity.

All the extra cash raised will be split between Diabetes UK and the British Heart Foundation. Contributing is voluntary – here's how it works:

  • At staffed checkouts, there will be a message at the end of the till explaining the scheme. Tesco says it'll usually be up to you to tell the cashier if you want to donate – though occasionally cashiers may ask.
  • At self-service checkouts, your till will display a short message before you pay explaining the scheme and asking if you want to donate. You'll have to choose 'Yes' or 'No'.

'Nobody asked me – every penny counts when you're a single mum'

Many have rightly pointed out that Diabetes UK and the British Heart Foundation are fantastic charities, and have praised Tesco for supporting them.

But since the scheme launched on Tuesday a number of Tesco shoppers have contacted MoneySavingExpert.com and taken to social media to say they were never asked if they wanted to donate and had no idea about the charity scheme until they checked their receipt.

Julia Hutchison, a single mum who works as a matron at a boarding house, told us her bill was rounded up after visiting a staffed checkout. She said: "It was only today when I was entering the receipt on my expenditure document that I noticed the 'round-up'.

"I was furious that I hadn't been asked. If I had been asked I may well have said 'yes', but how I spend my money has to be my choice. Nobody asked me, nothing was flagged to me. I am very careful with my money – that's why I subscribe to MoneySavingExpert. Every penny counts when you're a single mum."

Denise, who didn't give her second name, told us: "I visited a manned till and was not asked or informed at all. I noticed when I got back home. There was no notice about this anywhere that I could see and no notice about what the charity is. It may only be pennies but I give to charities of my choice.What if all shops added this charge?"

David, who also didn't want to give his surname, and who works as a support worker for vulnerable and disabled adults in London, told us one of his clients had to ask to have a 5p round-up removed after a cashier added it without it being requested. He said: "Tesco should know better – the same client has told me they no longer wish to shop there as they are worried about the whole situation."

Others have complained on Twitter:

What does Tesco say?

Yet even though bills are only being rounded up by pennies, Tesco is very clear this is an entirely voluntary contribution – you should only have your bill rounded up if you're aware of the charity scheme and agree to donate.

A Tesco spokesperson told us: "No money should be taken without someone's consent. If money has been taken without someone's consent we would rectify this and refund the money."

If your bill was rounded up without your permission Tesco says you should take your receipt back to the store and it'll refund you.