Spain's Ministry of Defense has approved the critical design review for Navantia's S-80 diesel-electric submarine, the company announced Friday.

The approval represents a milestone as it will involve the end of the boat's design stage and allows the shipbuilder to move into production, the state-owned Navantia said in a statement.

The sub's hull sections were expanded in April, the company said.

That work was required following the discovery roughly three years ago of issues with the boat's design, IHS Jane's reported in April.

The MOD design review approval demonstrates the program's technical feasability and the surmounting of past design problems, Navantia said.

The redesign brings a longer boat length and greater displacement.

Navantia's statement called the S-80 project the most complex engineering project being undertaken at the national level.