An Icelandic group has spent the past week building a stake in the retailer amounting to £170m.
The City said that over 13 million M&S shares changed hands yesterday, pushing the shares up 5.75p to 346.75p to finish the third biggest riser on the FTSE 100 Index.
Baugur is no stranger to retail stake-building. The company already has a stake in the Big Food Group, which owns supermarket chain Iceland and wholesaler Booker.
It is also part of a consortium bidding £1.2bn for Somerfield.
And beyond grocery it holds numerous stakes in high street clothing stores.
Two further Icelandic companies have also been thrown into the mix - banking groups Kaupthing and Landsbanki.
Any stake-building would be welcome for M&S which last month reported a 2.6% slip in like-for-like sales and a 19% slump in overall pre-tax profits.