Marks & Spencer has won an injunction blocking billionaire Philip Green's takeover bid before it is launched.
New M&S boss Stuart Rose is said to be behind the legal challenge
The injunction prevents City law firm Freshfields from advising Mr Green on the takeover bid - which he had been expected to launch on Thursday.
M&S claimed at the High Court in London that Freshfields should not be allowed to act against the retailer as it is a former client of the firm.
Mr Green has until 0930 GMT on Thursday to appeal against the decision.
In court, M&S lawyers Slaughter & May said Freshfields had a conflict of interest because it had advised M&S on a contract drawn up to bring clothes designer George Davies from Asda to M&S to design the Per Una range of clothes.
The M&S legal team argues that this gives Freshfields an inside knowledge of M&S.
The High Court decision could seriously delay Mr Green's bid.
All parties have declined to comment, but it is believed the legal challenge is fully supported by new M&S boss Stuart Rose.
Appointed on Monday, he is leading M&S's fight to repel any takeover bid unless it values the company at a significant premium.
Mr Rose replaced Roger Holmes, who was ditched by M&S in a swift response to news of the possible bid.
The struggling High Street icon is expected to face a bid of up to £10bn ($18.4bn) from Mr Green.
Mr Green, who owns clothing chain Bhs and retail group Arcadia, stunned the City last week after revealing he was considering a bid for M&S.
Mr Rose, boss of Arcadia before the Green takeover, also faces the challenge of turning around falling sales at M&S.
He began his retail career with a 17-year stint at M&S, and is credited with forging a turnaround at Arcadia, owner of the Top Shop and Dorothy Perkins clothing chains.