"Wedding season" sets the average adult back almost £800, a study found.
Research revealed the typical adult will attend three ceremonies this year, spending a total of £266 each time on accommodation, gifts, drinks and travel.
Grandparents will end up with the largest bill, around £430, while the best man will end up with a £313 dent in his bank balance.
The study also found four in 10 have gone over budget at a wedding.
However, the value of seeing loved ones walk down the aisle far outweighs the cost for many, with two thirds saying they feel it's a real honour to be asked to attend a wedding.
Paul Stokes of M&S Bank, who commissioned the study of 2,000 adults, said: "We know the privilege of attending a wedding far outweighs the cost for many of us, but with wedding season totalling nearly £800 on average, we wanted to look at the costs and see where guests could make some savings."
Buying gifts was one of the most expensive elements of attending a wedding, costing guests an average of £79 a time, with people most likely to spend more on their best friends than siblings.
Proud mothers of the happy couple will spend £307 on a new outfit, including more than £100 on a new dress, £51 on shoes, £30 on jewellery and £59 on hats and fascinators, as well as other accessories.
Paul Stokes added: "Despite the cost, being part of a loved one's special day is hugely important to most of us, so we wanted to show how guests can make the experience a little more affordable."
New M&S Current Account customers who switch using the Current Account Switch Service will receive a £125 M&S gift card, which can be used to cover the average cost of a wedding outfit, or a gift.