Seat‘s ultra economical Ibiza Ecomotive is experiencing none of the shortages afflicting its close relation the VW Polo Bluemotion – but there is a catch.

Fleets wanting the tax-busting version of the Spanish supermini will have to make do with the old model (pictured), and not the acclaimed new version.

The Polo Bluemotion and Ibiza Ecomotive share the same 1.4-litre diesel that’s been specially engineered to drop CO2 below 100g/km and into the zero road tax category. However, demand for the Polo version has been so high VW has had to restrict supply to fleets.

It’s that enthusiasm for the 74mpg diesel that has persuaded Seat to keep on the old Ibiza version until early 2009 – well past the July launch of the all-new five-door model.

The new Ibiza also goes on sale with petrol engines only, with the diesels and a replacement Ecomotive not due until next year.

Seat‘s head of fleet and business sales, Andy Watt, put a brave face on the TDI-free launch. “We’re not worried about the lack of diesels,” he said.

An Ecomotive version of the Leon hatchback is also in the pipeline, but Watt admits the market for sub-120g/km cars could grow more slowly following Ken Livingstone’s ousting. “We would have seen a significant uplift if Boris Johnson hadn’t become Mayor,” he said.

Johnson has said he will scrap the proposed free entry for sub-120g/km cars into the congestion charge zone.

Until the diesels arrive, no version of the new Ibiza will emit less than 130g/km, with the entry 1.2-litre petrol rated at 139g/km. However, Seat is confident the new Ibiza will make a much bigger impact on fleets than the old model.

“It underperformed in the fleet market for us,” said Watt. He blamed the lack of differentiation between five- and three-door models, reducing its appeal to downsizers.

The new one should change that, according to Watt: “The biggest area of growth will come from user-choosers,” he said.

The new Ibiza goes on sale on 16 July with a choice of 1.2, 1.4 or 1.6 petrol engines.

A three-door Sport Coupe version is due in November, while more powerful FR and Cupra models are also in the pipeline.