Monarch said its "newly designed non-reclining ergonomic aircraft seats" will also reduce fuel emissions because of the amount of weight they will save, while it is also hoped the move will bring an end to disagreements caused by passengers leaning back towards those behind them.
The announcement follows a survey by flight comparison website skyscanner, which found 91% of those asked wanted to see reclining seats on short haul flights either abolished or used only at set times.
Its poll last October also found that more than half of the 1,000 people questioned thought they should be restricted to set times on long-haul flights, with almost a third saying a reclined seat had caused them discomfort in the past.
A separate survey found that more than 60% of international cabin crew asked said they had been involved in or witness to an argument between passengers over reclining seats.
Monarch said that as well as giving passengers more legroom, the new thinner seats will also feature a tablet holder.
The airline said that when compared with a set of its existing seats, a set of new seats on a flight to Egypt saves 255kg fuel, equivalent to 816kg of carbon dioxide on a single flight.
Monarch's director of customer experience and marketing, Tim Williamson, said: "Our new ergonomic seats have been designed with our customers' needs in the forefront of our minds. The new non-reclining design gives our customers far more 'living space' than traditional seats, without the fear of the person in front impinging on their personal space.
"We noted from customer feedback that seat storage is particularly important. The new seats offer more flexibility than traditional 'pockets,' using the concept of the bungee cord the storage area can now comfortably fit water bottles, jackets and even kids' toys. The innovative tablet holder also enables our customers to create their own personal in-flight entertainment system."