A year ago the share of retailers offering such large discounts was just one in six, accounting giant Ernst & Young said this week.
But for this year's sales, item prices would be down 27 percent - a reduction one third more than last year, they said on Thursday. The firm questioned managers at around 120 various shops at the start of the month for their information.
Shoes, fashion and sports equipment should attract the largest discounts, with an average of nearly 30 percent off.
But home accessories, entertainment technology and household equipment will also be cheap, as will DIY products and even garden goods, with more than 20 percent price reductions to be expected.
"Business in clothing above all, was significantly worse than expected due to the cool and wet conditions in the spring and early summer," said Thomas Harms from Ernst & Young. This has left warehouses full of goods that need to be cleared to make way for new autumn and winter stocks.
The traditional start of the summer sales is July 22nd - Monday - but many shops have already smothered their windows with sales notices in a desperate bid to clear space and move products. Some products are being reduced by up to 70 percent.