Pressure is mounting on Tesco ahead of the publication of its interim results tomorrow, with new data showing the British supermarket giant is losing ground to indigenous Irish retailers Dunnes Stores and SuperValu.

Tesco saw its dominance of the Irish grocery market slip further in the 12 weeks to September 15th, with its market share now standing at 26.8 per cent, compared to a 28.7 per cent share this time last year, according to research company Kantar Worldpanel.

Dunnes Stores and SuperValu meanwhile bolstered their share of the multi-billion euro Irish grocery market and now hold a 22.1 per cent share and 19.7 per cent share respectively.

Kantar Worldpanel commercial director David Berry said Dunnes Stores has placed more emphasis on beating the competition this year, after losing the "Back to School" battle last year.

“Shoppers have been encouraged to spend more through a targeted voucher campaign, leading to a 6 per cent boost in larger, €50 plus shopping trips. This has helped to turn what was a €17 million loss from shoppers switching away from Dunnes last year into a €10 million gain this year,” he said.

German discount retailer Aldi and Lidl saw respective growth rates of 24.3 per cent and 13.1 per cent during the 12 weeks to September 15th.

Mr Berry said the build up to the Christmas period will show if the rival discounters can sustain their strong growth or if a ceiling is starting to be reached.

“Their ongoing growth, combined with the improved performance from Dunnes, has placed pressure on Tesco. Its sales have declined by 5.6 per cent, leading to a 1.9 percentage point drop in market share,” he added.

SuperValu managing director Martin Kelleher said the recent addition of Superquinn to the SuperValu network will create an "unrivalled national Irish retail brand" with one in four Irish consumers shopping at the expanded network of 222 stores.