Iceland raised prices faster than almost every other major UK supermarket in April, despite its chairman being appointed Labour’s cost of living tsar.

The frozen food retailer increased prices by 5.8 per cent in the four weeks to 19 April, according to Worldpanel figures seen by The Telegraph.

That was the second-highest inflation rate among UK supermarkets and above the sector average of 3.8 per cent for the period.

The rise was also almost double the official rate of food and drink inflation, which stood at 3 per cent in April, according to the Office for National Statistics.

The figures could prove awkward for Iceland chairman Lord Walker of Broxton, who was appointed by Labour earlier this year to help tackle rising household bills.

Lord Walker has been vocal in criticising retailers he believes are increasing prices too quickly, and recently warned petrol retailers not to “rip off households” or “take the p–s” by raising fuel prices following the spike in oil costs linked to the conflict in Iran.

Iceland disputed the Worldpanel figures and said they did not fairly reflect its pricing. The supermarket said the most recent published Worldpanel data for May showed its grocery inflation was below the wider market at 2.6 per cent.

The retailer also unveiled a fresh round of price cuts on Monday, with more than 200 frozen products to be discounted or added to deals from Tuesday.

Lord Walker said the April figures were based on “flawed methodology” and were distorted by specific lamb promotions from the previous year.

“This is flawed methodology, based on a small panel questionnaire, over a few weeks in April, a moment in time when Iceland had some very specific lamb deals in place last year, which distorts the comparison,” he said.

“It also does not take into account new products and deals in place this year.

“We tirelessly track prices, investing where customers need it most, to ensure our prices are consistently lower than the market. Families are still under real pressure, which is why we are putting more money into deals, lower prices and value across frozen food and everyday favourites.”

A spokesman for Iceland added that its own pricing data showed the retailer remained “highly competitive” on the products and ranges most important to shoppers.

“We are continuing to invest in deals, lower prices and value across the foods families rely on,” the spokesman said.

“Customers are still under real pressure from food costs, so our priority is simple, helping them get more from every shop.”

The figures come amid heightened scrutiny of retailer pricing, with the government under pressure to ease the burden on households.

Lord Walker’s role sees him work with a Downing Street “sprint team” focused on finding ways to lower household bills.

However, the Competition and Markets Authority said on Monday that it had found no evidence petrol retailers had opportunistically used the fuel crisis to boost profits.

The watchdog said higher oil prices explained most of the increase at the pumps in March and April, although it warned that weak competition in the fuel sector remained a concern.

Chancellor Rachel Reeves has also accused some businesses of profiteering and “price-gouging”, sparking tensions between retailers and the government in recent months.

The Treasury was forced to postpone a meeting with supermarket bosses in March after only one grocery chief, Morrisons boss Rami Baitiéh, agreed to attend.

Asda chairman Allan Leighton pushed back against accusations of profiteering at the time, saying they had “zero credibility”.

Concerns are now growing over food prices, with higher fuel costs expected to feed into supermarket inflation by increasing transport, fertiliser and feed costs.

The Food and Drink Federation has warned that food inflation could climb as high as 10 per cent later this year.