Morrisons has rejected a buyout offer from US private equity firm Clayton, Dubilier & Rice (CD&R).
The UK's fourth-largest supermarket group said the offer "significantly undervalues the business".
CD&R earlier said it noted "the press speculation regarding a potential transaction" and it confirmed it was considering a possible offer.
It has previously made investments in the discount shop chain B&M, from which it made more than £1bn.
Morrisons said it rejected a conditional cash offer from CD&R of 230 pence per share, which amounts to just over £5.5bn.
In a statement, the supermarket chain said the board had evaluated CD&R's conditional proposal together with its financial adviser, Rothschild & Co.
They had "unanimously concluded" the offer "significantly undervalued Morrisons and its future prospects" and therefore rejected it.
Last month, Morrisons said sales had increased by 2.7% in the 14 weeks to 9 May. But in the previous three months alone it had faced a £27m bill for Covid-related costs.
The move by CD&R, one of the biggest takeover firms in the world, would have been one of the most high profile of many bids for UK companies in the past year.
BBC business correspondent Katie Prescott said the flurry of takeover activity was being fuelled by relatively low share prices of businesses in the UK compared to abroad and cheap money because of low interest rates.
Morrisons - with its £1bn property portfolio, most of which it owns - and its 10% of the grocery market - is an attractive proposition.
There has been speculation about bids for Morrisons in the past from Amazon, which sells the supermarket's products on its site.