France and Australia have signed off on the long-awaited agreement to build 12 advanced subs for the Royal Australian Navy, a half century deal worth more than A$50 billion.

Under the Strategic Partnership Agreement (SPA), 12 conventionally powered submarines derived from the Naval Group Barracuda nuclear design will be constructed in the new shipyard now under construction in Osborne, South Australia.

The signing ceremony was attended by Prime Minister Scott Morrison, Defence Minister Christopher Pyne, France’s Armed Forces Minister Florence Parly, South Australian premier Steven Marshall, plus senior Defence and industry figures.

The SEA 1000 Future Submarines project is Australia’s largest ever defence procurement project, and the largest ever foreign sales deal for the French shipbuilder.

The signing was an occasion for much reflection on the strength of the relationship between Australia and France. “I reckon it’s the greatest partnership between Australia and France since Nancy Wake was let loose on the Nazis in the Second World War,” the Prime Minister said.

Mr Morrison said the signing of the SPA underscored the longstanding and strategic partnership between Australia and France. “A spirit of defence cooperation and collaboration between our nations reaches right back to the first world war. This program brings new depth to that partnership through a multi-decade program to build and sustain these submarines in Australia.

“It is more than a contract. This is a project that will not only Australians safe. It will deliver Australian jobs. It will build Australian skills.”

Ms Parly, who flew to Australia after visiting French forces assisting in the fight against Islamic State remnants, said the 12 Attack-class submarines would be a world-class capability.

“In strategic terms it means Australia will have an edge over the entire region. In industrial terms it means a lot too. Australia will revive a sovereign naval industrial and technological base to build a 4,000-tonne sub marine and sustain a fleet of 12 units,” she said.

Protracted negotiation of the SPA prompted some speculation that the project was in trouble. Ms Parly said she was aware of this speculation.

“I thought open your eyes – this is the deal of the century,” she said. “This is a partnership for a century. We were right to take the time necessary to dot all ‘I’s and cross all ‘T’s. We want to leave our successors a perfectly sound project and I believe this is what we will hand over to them.

“It takes a lot of confidence for Australia to bet on France and a lot of confidence for France to share with Australia the capability that is so close to the core of our sovereignty and our strategic autonomy and a result of immense investment over decades.”

Naval Group chairman and chief executive Herve Guillou said Australia chose the French submarine design because of its endurance, long range and acoustic superiority, and France’s ability deliver this technology.

He told ADBR it was not unusual for negotiations on a project of this size to take as long as they did. “You need to make it right the first time and if it takes a bit longer, it takes longer,” he said.

Construction of the new submarine construction yard at Osborne started in December. Mr Guillou said the first steel would be cut around 2025 or 2026.

“The first ship should be sailing in the early 30s, the last one in the early 40s and it should be decommissioned probably in the early 80s, we are looking at a very very long term partnership with Australia,” he added.

“Naval Group is known for building world-leading, technologically advanced submarines and has built 100 of them for nine different countries. This agreement with Australia will see Naval Group transfer the ‘know-how’ and ‘know-why’ to Australia to become a sovereign submarine nation.”