Tesco has made changes to its Clubcard loyalty scheme - dropping the value of some rewards and raising some others.
Shoppers collect points for money spent with the supermarket giant and these then become vouchers which can be used for things such as restaurant meals.
Some could be used for four times or two times their value, but to "make things easier" Tesco said most are changing to three times their value.
But the move provoked an angry response from customers on social media.
Tesco emailed customers on Monday to tell them of the change, which was introduced immediately.
On its website, it said it was "simplifying" the rewards available so people did not "have to keep checking whether you're getting two times or four times the value depending on which reward you're claiming".
A spokesperson added: "This builds on the work we did last year to further help customers, when we introduced the new contactless Clubcard and made significant improvements to the Tesco Clubcard app."
While the retailer said any orders already placed for vouchers would be honoured, it did not give any time for customers to cash in vouchers which they had been saving.
Some of the rewards dropping in value are for popular restaurant chains and days out.
One shopper said he had in effect lost £100 as a result of the change.
As I have £100 in Clubcard coupons that were "worth" £400 in vouchers yesterday and are now only worth £300 I feel like I have just been a victim of a pickpocket. Your headline "major changes" is woefully inadequate. This is scandalous #tescoclubcard #tesco— Stephen Ellerington (@s_ellerington) January 15, 2018
However, Tesco defended its move. "The vast majority of Clubcard customers redeem their vouchers at face value, rather than use the rewards scheme, so this change will only affect a small minority of total Clubcard customers," a spokesman told the BBC.
But Anika Newjoto, editor of the Shopper Points loyalty points website, said: "Making these changes with no notice stinks.
"Tesco Clubcard vouchers have a two-year expiry date. Many shoppers collect them over a longer period to pay for a large transaction.
"Loyalty schemes are meant to work two ways, not one way. If a shopper gives their business to a store based on the promise of certain rewards, it is totally unfair for those rewards to be taken away or reduced in value with absolutely no notice."
Martin Lewis, founder of the Money Saving Expert website, called on Tesco to rethink its decision, especially its immediate introduction.
Calling the move "a slap in the face to hundreds of thousands of loyal customers", he said: "This is a terrible policy implementation by Tesco.
"I call on it to urgently review its policy to give a window of opportunity to those who've already saved substantial points, and will lose out by these changes, to still redeem at the prior value."
Tesco is not the first supermarket to cut back on the benefits of its reward scheme.
It introduced Clubcard in 1995 and within a year, customers were spending 28% more in its stores and 16% less with its closest rival Sainsbury's, which by then had introduced its own Reward card.
Sainsbury's moved over to the Nectar card scheme, in conjunction with other companies, in 2002 and in 2015 it halved the reward points available to its card holders.
Annich McIntosh, editor of Loyalty Magazine, told the BBC that loyalty schemes were "at an eight-way interchange".
"So many different things are happening in loyalty schemes and the supermarkets are in competition with online companies," she said.
"Amazon do things with Prime that is really loyalty but don't call it loyalty. They are very clever and tie the customer in to such an extent that they think they can't do without it because of all the things they are getting.
"Customer loyalty is at the basis of all business and that isn't going to change. It's how people persuade you to be loyal that is up for major change and this could be a very exciting year for the sector."
She added: "Any change to these programmes causes a row. When Avios changed a few years ago, there was a furore online but then later I heard people saying it was actually better for them."