From the monster Dreadnoughts of the Edwardian era to the Royal Navy's 21st-century helicopter carriers, Clyde yards have launched some massive ships.

However, now only one yard left on the river, Govan, has the ability to build the world's biggest vessels, military or commercial. And it, as The Herald revealed this week, is now under threat.

Defence giant BAE Systems has revealed its preferred option is to focus all of its surface shipbuilding at a single £200m "frigate factory" across the Clyde at Scotstoun.

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However, last night the company was still unable to say it would replicate the kind of capacity it has at Govan to make major capital warships

Industry insiders are increasingly worried neither BAE Systems nor its main customer, the UK Ministry of Defence, have committed themselves to having a so-called Panamax-sized construction dock at Scotstoun. This is a dock capable of building Panamax ships, the vessels whose size is limited only by whether they can get through the Panama canal.

A spokeswoman for BAE Systems said: "Work to mature our facilities investment proposals in Glasgow is ongoing.

"As part of this, we are assessing the best configuration of the facilities, including the potential size of dock required within a new state-of-the-art facility at Scotstoun to ensure we offer value for money to our customers and have the flexibility to remain competitive in the long term."

Govan has had a Panamax dock since the early 1990s when its then operators, Kvaerner, built the facility for the giant bulk carriers it used to make at the site.

The dock has since been used to make ships such as the Royal Navy "helicopter platform" or amphibious assault ship HMS Ocean, which is 35 metres wide at its beam.

BAE has started consulting on major facilities investments at Scotstoun as it prepares for an expected order of 13 Type 26 frigates from the Royal Navy. An announcement confirming the order - and whether it will be met by building at a single Scotstoun site or at a combination of both Clyde yards - will not be made until after the independence referendum.

The company has announced outline plans for a modern dock hall to cover the berth, keeping its hi-tech weapons systems well away from prying eyes. This would be 330m long and nearly 50m wide.

The company is understood still to be haggling with the UK Government to make the Scotstoun dock under that shed at least big enough to build another HMS Ocean. Sources stress that is far from the case in all options under consideration.

John Dolan, GMB's representative at Scotstoun, said: "The company wants to it to be wide enough to build two frigates at once, one for the Royal Navy and one, hopefully, for export."

Industry sources have suggested the Scottish Government may have to step in and up the MoD's contribution to the dock, to "futureproof" Scotstoun so that it, like Govan, can make ships bigger than frigates in the future.

Deputy First Minister Nicola Sturgeon wants to see both yards saved.

Her spokesman said: "The Government and its agencies stand ready to support the company to help secure its long-term future on the Clyde. We very much hope for a two-yard option.

"However, the UK Government's apparent inability to secure the future of Govan highlights the hypocrisy of the No campaign.

"It said a Yes vote threatened Govan, but the fact is that a plan for its closure is happening now - on Westminster's watch."