Last year the discounters were the darlings of the grocery industry and Aldi was enjoying sales growth of more than 20%.

But competition has got tougher. The supermarkets have gone promotions crazy and Tesco even declared itself Britain's biggest discounter.

Nielsen now reckons the discounters' growth will drop to 10% or less over the next six months, roughly on a par with the big four.

Despite this sobering forecast, Aldi has not dropped its plans to open a store a week. But it has decided that it wants its suppliers to help pay for its expansion. It has sent out an uncharacteristically blunt letter to suppliers demanding a 5% reduction in prices.

It's a move that has shocked many suppliers because Aldi is renowned for treating them fairly. As one said: "Aldi's halo has finally slipped."

It seems that, as the supermarkets have started stealing Aldi's retail tactics, Aldi is stealing their hardball manner of dealing with suppliers.

With Aldi battling to keep prices low and quality high while increasing its estate, much depends on how much slack suppliers will cut it.