Make Your Mark launches on Monday, providing unpaid month-long placements, with up to 37 hours a week spent either in training or on the shop floor.
Those who complete placements also have the chance to work full time at M&S, if they perform well.
It builds on the retailer's existing Marks & Start programme that has offered placements to more than 6,000 participants since launch in 2004.
Marc Bolland, chief executive at Marks & Spencer, said: "Youth unemployment has become one of today's key social issues and we want to lead the charge for businesses to do more to tackle it head on.
"That's why we've set aside 1,400 new placements for young people in our stores and have called on our suppliers and other businesses to start engaging their communities and make their own commitments to youth opportunity."
Make Your Mark sees M&S expand its previous placement programme, which took on 1,100 young people last year.
Under Make Your Mark, if a job vacancy is not available at M&S following a placement or if further support is needed, The Prince's Trust will provide help for six months to find an alternative.