The Spanish Navy's Isaac Peral submarine, part of the new S-80 series commissioned from state-owned shipbuilder Navantia, has a serious problem — it is between 75 and 100 metric tons too heavy.

The excess — the result of miscalculations at the engineering stage — may not sound like much compared with the sub's 2,200-ton weight when floating and 2,430 tons when submerged, but could compromise its ability to submerge and re-emerge.

Navantia, which has never built a submarine before, has admitted the existence of "deviations related to the balance of weights," as reported by La Verdad de Murcia, and estimated that correcting the problem will mean a delay of 12 to 24 months on the March 2015 delivery date. The firm says it is planning to bring in a foreign advisor to solve the problem.

Nobody has yet dared to calculate how much the changes will add to the project, which at 2.2 billion euros for four subs is already one of the most expensive in Spanish military history.