A sharp rise in food prices has added £21.31 to the average household shopping bill over the past three months as the number of promotions fell to an 11-year low, according to new grocery market data.

The price of everyday goods at supermarkets rose 2.3% in the three months to 26 March from a year ago, said Kantar Worldpanel, a sharp pick-up from the 0.2% food price inflation recorded in the 12 weeks to 1 January – the first time prices rose in more than two years.

Rising prices for everyday staples such as butter, fish, tea and skincare have been partially offset by falling prices for crisps, bacon, chocolate and fresh poultry, Kantar said.

Fraser McKevitt, head of retail and consumer insight at Kantar, said: “We expect inflation to continue to accelerate, and as a result we’re likely to see consumers looking for cheaper alternatives. A reduction in promotional activity means the proportion of spending on promotions now stands at just 32.9% – 5.5 percentage points lower than last year.”

Supermarkets have been cutting multi-buy deals and other special offers, after mounting evidence that they confuse shoppers and increase food waste. Separate data from Nielsen showed the proportion of consumer spending on promotions at UK supermarkets has fallen to an 11-year low.

Just over a quarter (26%) of money spent at supermarkets went on products with temporary price cuts or multibuy offers in the four weeks to 25 March, the lowest level since 2006. Supermarkets’ own-label products are more affected than other categories: 18% of sales went on promotional spending, compared with 41% of branded goods sales.

“The level of promotional spend has gone back to levels not seen since before the 2008-09 economic crisis,” said Mike Watkins, Nielsen’s UK head of retailer and business insight.

“The last few years have seen about a third of the typical supermarket shopping bill going on promotional items. However, to be more price competitive, supermarkets have turned temporary price reductions into permanent cuts, so there’s less promotional activity as many prices are cheaper all-year round.”

Last year, Sainsbury’s became the first of the major supermarkets to phase out controversial “buy 1, get 1 free” offers in favour of lower everyday prices.

However, the sharp slide in the pound since the Brexit vote last June has increased the cost of imported goods, and grocers have been passing on the rises to customers.

Kantar said supermarket sales of “free from” products grew by 36% year on year in the past three months, with more than half the population (54%) purchasing a gluten or dairy-free product. That’s 3.3m more people than last year, led by younger shoppers keen to eat healthily.

Supermarket sales and market share

Photograph: Kantar Worldpanel

German discounters Lidl and Aldi have reached new record market shares and together account for 11.7% of the grocery market, according to Kantar. Lidl was the fastest-growing retailer with sales growth of 15%, taking its market share to 4.9%. Aldi increased sales by 14.3%, taking its share to 6.8%.

Iceland posted its strongest sales growth since March 2013, of 9.8%, after introducing more fresh and chilled lines. Bigger rivals Tesco, Sainsbury’s and Asda have seen sales and market share slip. Morrisons was the only one of the big four to achieve higher sales, of 0.3% in the past 12 weeks.