Land Rover is planning to double production of the iconic Defender when it replaces the go-anywhere model in 2013.

At least an extra 25,000 units of annual production on top of the current average of 23,000 to 25,000 civilian Defenders is reckoned to be necessary to justify the investment in an all-new model, according to senior Land Rover sources.

Although the new model is six years away from Job One, Land Rover has internal teams working on the scope of the new Defender programme.

Already clear is that the new Defender will have to be built on a dedicated chassis platform. Land Rover might have been expected to share components with the T5 'integrated bodyframe' structure from the Discovery 3, but highly-placed sources confirm it is too costly to underpin a utility model like Defender.

Another is that the Defender is headed back to North America, a market vacated in the mid-90s when airbag legislation killed sales. Even the latest 2007MY Defender, launched last week, doesn't have airbags.

Also known is that new Defender will retain rugged go-anywhere durability and the styling will be an evolution of today's look, influenced by Gerry McGovern's new 'premium adventure' styling theme.

Emissions legislation, however, will catch up with the Defender in 2013. Interestingly, Land Rover is confident that it can get the Defender through the next hurdle of 'ped-pro' (pedestrian crash protection) laws, due in 2010.

"We're pretty sure we've got a solution to that," said managing director Phil Popham.

For the replacement, one intriguing possibility is for Defender to share development costs with the next generation Ford Transit, also due for replacement early next decade. In many ways the Defender is closer to Ford's light truck than Land Rover's increasingly up-market models.

The Transit is made in a huge variety of models, including front and rear-drive with in-line and transverse engines and both monocoque and body-on frame construction.

Land Rover's latest version of the Defender has already moved in that direction, borrowing the Transit's 2.5-litre four-cylinder diesel and six-speed gearbox , to which are added a low-ratio transfer box for off-road 4x4 performance.

The Transit's truck-like flexibility of manufacture also fits the Defender's profile of 170 different variants.

Many of these variants are more in the realm of commercial vehicles, featuring specialised bodies, extra hydraulics and other add-ons to the traditional body-on-frame chassis. Although profitable, these alternatives have to be made in a separate shop or farmed out to specialist body builders.

The main challenge facing Land Rover product planners of the Defender replacement is to set ride, handling and refinement targets for a new chassis platform that will generate higher sales around the world without interfering with specialist and military sales.

This product planning conundrum has been visited many times before, both by BMW in the mid-90s, when it owned Land Rover, and, in recent years, by Ford.

In fact Ford's original product plan for Land Rover had the Defender replaced in 2005. But the Range Rover Sport derivative was switched in the cycle plan for the venerable Land Rover.

Julian Rendell