Aldi has expressed its "surprise and disappointment" after the ASA banned its 'Swap and Save over 40%' email ad.

A complaint to the ASA suggested the ad was unfair and misleading after it compared the price of Aldi's own-label products against the price of premium brands at other supermarkets.

The ASA upheld the complaint after concluding that Aldi had not used a like-for-like comparison, and should have compared its own-label products with other supermarkets' own-label products. It also failed to identify which retailers were used in the comparison.

Aldi must ensure that future price comparisons compare its own-label products with supermarket own-labels, and products should be identical sizes, the ASA ruled. Other retailers should also be identified and individual prices should be shown for each supermarket.

A spokesman for Aldi said it would appeal, insisting Aldi's products could be fairly compared with premium own label.

"It was always our intention to produce an ad that clearly showed consumers they have a choice about the prices they pay for products," he said.

"We ­believe our ad did this and included fair, conservative price comparisons ­using a credible, independent source []."