The BMW i8 Spyder is presenting "major challenges" to engineer, company officials have told Autocar.

A spokesman said that incorporating the carbonfibre-reinforced plastic bodyshell construction of the i8 into a convertible format was proving problematic: "The last architecture presented some fairly major challenges, to be able to produce a convertible in that format. The rigidity was hard to find with that particular architecture."

The spokesman also confrmed there is "no set time frame" for the market introduction of the i8 Spyder.

That contradicts media reports which have stated a late 2015 launch for the i8 Spyder. A BMW i forum reports that BMW officials gave the green light to the project last week after seeing a late-stage development prototype. 

The i8 Spyder will reportedly use the same hybrid electric powertrain as the i8 coupé. That means a three-cylinder engine and generator mounted behind the rear seats and driving the rear wheels, with an electric motor driving the front wheels. In total, the i8 coupé develops 228bhp, with BMW claiming it can hit 62mph in 4.4 seconds.

At the concept's debut at the 2012 Beijing motor show, BMW said the i8 Spyder would make production in 2015. These development challenges would appear to threaten that timeframe.