TUI Travel's fleet-renewal agreement covers up to 150 Boeing 737 Max jets, including an initial firm batch of 40 Max 8s and 20 larger Max 9s.
The holiday company has agreed to take options on another 60 while 30 more are the subject of purchase rights.
All Max variants are powered by the CFM International Leap-1B engine. The firm part of the order, if approved, would require eight spare powerplants in addition to those fitted to the 60 twinjets.
Although the list price of the aircraft is $6.1 billion, TUI says it has secured a "significant discount" in the form of concessions and support.
TUI has six airlines in its holiday operation, which have a combined fleet of 141 jets, and all but its French long-haul operator Corsair use single-aisle aircraft, primarily 737 variants.
Thomson Airways, TUIfly and Jetairfly have some 85 737s between them while another dozen or so are deployed with TUIfly Nordic and ArkeFly.
UK operator Thomson - which is scheduled to receive its first Boeing 787 today - also operates 757s in its narrowbody fleet.
TUI Travel says its overall single-aisle fleet amounts to 114 aircraft and that these transport some 80% of the holiday company's passengers.
"In order to maintain cost competitiveness and support TUI Travel's aim to minimise the environmental impact of its activities, the existing [single-aisle] aircraft will need to be replaced in the next decade," it adds.
"The proposed transaction will ensure that the group has sufficient aircraft to fulfil its long-term plans."
Shareholders have yet to approve the agreement but, if cleared, deliveries of the 737 Max jets will run from January 2018, the year after its entry into service, to March 2023.