Attempts by Aldi, Lidl and Netto to expand could be hampered by a Competition Commission proposal to introduce a 'competition test' for new store developments.

The idea, mooted in the provisional findings, would mean any proposed new store would undergo a test, possibly administered by a specialist body such as the OFT, that would decide whether it would provide "effective competition" to incumbent grocery retailers in an area.

The test would also apply if any divestment of stores or landbanks were ordered.

However, the big three discounters could stand to lose out because the commission has decided they do not represent effective competition for the bigger supermarkets.

A spokesman for the commission admitted the proposals could "slightly disadvantage" discounters. "In areas where we have worries about concentration, we would seek a retailer that matched on both price and range for any new or divested site. But how any test would work is something we'll have to work out - for example who should be included and how we calculate it."

All three discounters declined to comment. However, the news is likely to disappoint Aldi and Lidl. In their submissions to the inquiry, they made it clear they considered themselves one-stop shop retailers in competition with the big four. One retailer likely to be happier is M&S, which the commission has decided should be considered effective competition to the main supermarket chains.