Retail tycoon Philip Green has been set a deadline of August 6 to make a formal offer for high street giant Marks & Spencer Plc (M&S) or be prevented from bidding for the group for six months, the UK Takeover Panel ruled today.

The merger and acquisition regulator said its ruling followed representations made by M&S advisors and talks with advisors representing the retailer and Green's investment vehicle Revival Acquisitions.

"No extensions to this (noon) deadline will be granted, except with the consent of the Panel executive," the panel said in a statement to the London Stock Exchange.

M&S's board has so far turned down two informal offers from Green for the group, saying they undervalued the retailer.

Green, the owner of fashion retailers Bhs and Arcadia, tabled a revised 370 pence per share offer for M&S on June 16, valuing the company at £8.14 billion.