Everyday food prices rose by 8% on average over the year to June based on new method being trialled to measure inflation

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Consumers might not have consistently benefited from supermarket price wars after all as the cost of basic food and non-alcoholic drinks rose much faster than previously thought over the past year, based on new data.

According to a new way of measuring the cost of living being trialled by the Office for National Statistics, the price of everyday items such as milk, bananas, potatoes and tea bags soared by an average of 8 per cent over the year to the end of June.

Some items showed even biggest rises, with a kilogram of cheddar cheese up 15 per cent and a 500g packet of spaghetti rising 19 per cent.

Prices gone bananas: New data shows the cost of basic items might have rose faster than previously thought

The figures are in contrast with the traditional consumer price index measure of inflation, which has been near zero in recent months and was at 0.1 per cent in July thanks to falling food and fuel prices.

However, the ONS said the two methods used to calculate the increase in the cost of living were not easily comparable.

The new method is based on ‘scraping’ prices from supermarkets’ websites every day for the 35 items in the ‘basket of goods’ - the list of products used to calculate inflation – and only scans Tesco, Sainsbury’s and Waitrose.

This rules out several supermarket chains that don’t have a website, such as dominant discounters Aldi or Lidl, which grown sharply to take a reasonably large share of the grocery market recently.

Around 6,500 prices are collected each day, which adds up to roughly 200,000 a month, a much larger number that than gathered by the traditional approach used for calculating the CPI rate.

For example, while the CPI is based on 140 prices for a bottle of whisky on a specific day each month, the new method draws on over 6,000 quotes over the course of every calendar month.

Big variations: The graph shows how prices rose faster on a monthly basis according to the 'chain index'

However, the new method shows big variations between prices collected every day - what is known as a ‘chain index’ - and the average cost across a fixed time period, known as the ‘unit price’.

While the chain index for the 12 months to June suggests a rise in everyday food and non-alcoholic drink of 7.9 per cent, the unit price suggests an increase of between 1.3 per cent and 1.8 per cent.

The cost of a small individual yoghurt rose by 13 per cent according to the weekly chain index, but fell between 3 per cent when measured by unit price.

And while the cost of one packet of 80 tea bags went down by 4 per cent on the day-to-day measure, the unit price showed a rise of just under 3 per cent.

The difference is due to the way the unit price is calculated, which discounts data for items when a price is not available in all periods of comparison - for example, if a supermarket website fails to display a price on a particular day.

This ‘significantly reduces the number of prices used in the calculation of the index,’ according to the ONS.

But the inconsistency between the different measurements could mean that shoppers across the country are seeing a greater variety of day-to-day prices rises and special offers than those reflected in monthly figures, including the official rate of inflation.

This would support the findings of a recent report by the Institute for Fiscal Studies which called for more data sources to be used in the compilation of consumer price statistics, including those from supermarket websites.