A sports bra maker and three major department stores are being investigated by the trading watchdog over allegations they ­illegally agreed to prop up prices.

The claims relate to the Shock Absorber bra, once advertised in a campaign featuring tennis player Anna Kournikova.

The Office of Fair Trading (OFT) alleges that lingerie maker DB Apparel UK entered into anti-competitive agreements with John Lewis, Debenhams and House of Fraser between 2008 and 2011, designed to increase the retail price of Shock Absorber bras.

The regulator said its provisional view was that each of the alleged nine agreements amounted to resale price maintenance, in a "serious infringement of competition law".

Resale price maintenance occurs when retailers agree to a re-sale price set by ­product suppliers.