Entrepreneur Philip Green has been given until midday on August 6 to make a formal offer for Marks & Spencer.
Green has made two informal offers to M&S since he initially voiced his interest in May. Both were rejected by the group's board.
The latest £8.4 billion bid fell significantly short of the £9bn shareholders evaluation.
"No extension to the deadline will be granted, except with the consent of panel executive," said the panel.
A failure by Green to submit a bid by the deadline will put him out of the running for the takeover for six months.
