The food retailer will be giving away a free lunch every 12 seconds this September.
Mains on offer include sandwiches, wraps, salads and even sushi.
The supermarket will also throw in a drink, from juice to water to a fizzy pop.
This could take the total value of the meal up to £5.50, depending on the combination you choose.
Speaking about the giveaway, M&S Buyer Matthew Pudney said: ‘The combination of main and drink options is endless!
“If you are lucky enough to be one of our winners, you can treat yourself to a classic top seller like the BLT or Prawn Mayo or mix things up with a delicious California Sushi Roll Selection.
“There are plenty of gluten free, vegetarian and vegan options to choose from too – meaning there is sure to be something for everyone.”
So how do you take advantage of this great freebie?
Free lunches will be given to those who find a hidden special voucher inside a Marks and Spencer sandwich.
The hidden tickets will be available from the 18th September until the 1st October.
Lucky winners will be able to redeem their ticket from 18th September until the 14th October.
If you don’t win a free meal, there is still the option to get a meal deal for good value, after M&S launched a lunchtime offer for just £3.50.
The luxury food favourite have a brand new, revamped bundle offer rolling out in stores.
As well as classics such as chicken and avocado, more unusual flavours such as brie, grape and cranberry will also be included.
Unlike other supermarkets, there will be a wide selection of gluten free choices too
The three-part bundle includes one of M&S’ famous sandwiches (including gluten free versions), a bag of hand cooked crisps or popcorn, plus a drink, with options including bottles of Coca-cola and fresh Florida orange juice, which would usually set you back £4.35.