BAE Systems has unveiled detailed plans for a £200 million "frigate factory" that it hopes will secure shipbuilding on the Clyde for decades.

The defence giant has given its clearest signal yet that it aims to build a new generation of Royal Navy warships at a proposed "world-class facility" at its Scotstoun yard in Glasgow.

However, bosses admit their preferred scheme would cast doubts on the long-term future of their second Clyde yard, Fairfields in Govan, which they now describe as "sub-optimal".

BAE Systems and the UK Government will not announce a final decision on the factory or the new Type 26 frigates it will make until shortly after September's independence referendum.

Ministry of Defence officials will have a huge say over whether BAE Systems builds the ships in Scotland and whether it does so at the proposed new facility in Scotstoun or, under a Plan B, across both Glasgow yards.

However, BAE Systems director of business and transformation Charlie Blakemore has made little secret of the fact that he would like to see the Type 26s built under a single roof in Scotstoun.

He said: "The Scotstoun facility requires more investment but delivers significantly more benefits.

"There is an affordability issue in the initial stages, more construction needed, more ­facilities work, but it will provide a capacity that is world-class. We will be able to compete in a more level playing field."

Mr Blakemore said investment at Scotstoun would include a 360 yard-long new dock hall capable of building two ships at once - all indoors. Such a facility would churn out warships one-third faster than across both yards, making the navy's new Type 26s "significantly cheaper".

It would put BAE Systems in a position not only to produce ­frigates for the Royal Navy but to offer competitive terms to foreign navies too.

The company has not ruled out Mr Blakemore's Plan B. This would cost £100m in ­investment and involve building partly in the open and shipping half-built vessels up and down the Clyde in barges from yard to yard.

Unions have cautiously backed the proposals for expanding ­Scotstoun and, potentially, ­phasing out Govan.

BAE Systems will need the same number of workers to make its ships however it organises production. However, GMB steward John Dolan said the Scotstoun option offered the best long-term future.

The Labour stalwart said: "If we stand still, we will fail in five years.

"The facilities at Scotstoun would be first-class and would make us a more modern 21st century yard able to compete with foreign shipbuilders. "

Mr Dolan and pro-union ­politicians have long argued the Type 26 contract will only come to Scotland if there is a No vote.

Industry insiders stress making the frigates in England would involve an expensive operation to reskill and retool the Portsmouth yard, currently slated for closure.

Mr Blakemore has stressed that his firm's decision to earmark the Clyde for the frigate contracts had nothing to do with the vote.

He said: "The reason we have chosen the Clyde is because of the skills and the capabilities. This is all based on industrial reasons, not politics."

Asked if BAE Systems could make the Type 26s in England if Scotland votes Yes, he said: "Anything is possible. It is just there is going to be a cost. It's an option we have not explored in great detail.

"We do have the same skill sets in Portsmouth as we have here. They are just not available in the same depth and numbers."

The SNP argues it would be impractical for the Royal Navy to insist its warships are made in England after independence.

Deputy First Minister Nicola Sturgeon welcomed the proposed Scotstoun investment, but insisted Govan must be safeguarded.

She said: "It is ironic that we have Better Together leafleting in Govan saying vote No to save Govan."