Concerns over jobs at the Govan shipyard have been raised with the Prime Minister, Nicola Sturgeon has said.

The First Minister has wrote to the Prime Minister seeking a "cast-iron commitment" that MoD contracts will delivered as originally planned. Trade unions have expressed worry that extending the period of construction or delaying the starting point of the Royal Navy's new type-26 frigates will lead to job losses at the site.

Sturgeon said: "Scotland's shipbuilding industry has a proud history. The workers on the Clyde have unparalleled skills and experience - and they deserve better than empty promises from a Prime Minister who has already admitted that he "doesn't care" about Scotland.

"Workers won a contract for 13 type-26 frigates and have already had to watch that be cut back to only eight vessels. Now there are doubts over the time table for these vessels and for other work due to come to the Clyde.

"The workforce here has already seen its numbers reduced and it would be a disgrace if they are to face further cuts.

She continued: "A series of claims were made by the Tories and Labour during the referendum about employment at these yards and we will do everything in our power to hold the Tories to their promises.

"We already know that David Cameron is trying to avoid Scotland during this election campaign - but he can't avoid his obligation to the workers on the Clyde. And he owes it to them to give a cast-iron commitment that this contract will be delivered in full, and on the promised timescale.

Last week union chief Gary Smith said that any change in planned operations would be a "betrayal" to the workers.

The GMB Scotland secretary said: "This would be a total betrayal of the upper Clyde workforce by a desperate Tory government trying to shift the goalposts in the face of their failing economic stewardship.

"We've gone from the upper Clyde workforce being promised the manufacture of 13 type-26 frigates in 2014, only for that to be cut to eight frigates last year. We've gone from the promise of a world-class 'frigate factory' on the Scotstoun site, only to be told the plans were shelved.

"We've gone from promised investment that would secure thousands of skilled jobs and hundreds of apprenticeships for a generation, only to be told to prepare for redundancies. In the same week that Scottish unemployment increased by 20,000, the prospect of significant job losses at Govan and Scotstoun doesn't even bear thinking about - it could tip our economy over the edge."

Scottish Labour leader Kezia Dugdale also said that any job losses would be a betrayl to the Clyde workers.

Writing in the Daily Record, Dugdale said: "As they work on the third of five offshore patrol vessels, everyone is gearing up for the type-26 frigates which will support jobs at the yard.

"That's why the suggestion at the end of last week - after my visit - is such a blow for the workforce there. Reports suggested that the frigates may be delayed by up to a year and the upgrading of the facilities on the Clyde may be abandoned.

"With the offshore patrol vehicles still providing work for the yards, there isn't an immediate threat of job losses, but we need to know urgently from the Government what their new plans are for the timetable and whether there is going to be any gap between the current contracts ending and the type-26 frigate beginning.

"We also need to know that the Government remains committed to building at least 13 type-26 frigates on the Clyde and that the promised upgrade of the facilities there will be carried through. This is important because we don't just need to see the jobs retained now, but the promise of investment met so that Clyde shipbuilding can have a long and certain future.

"If the Tory Government delivers anything short of what they promised then it will be a deep betrayal to the workers on the Clyde and their families."

Scottish Liberal Democrat leader Willie Rennie said that there are "serious concerns" about the orders

Rennie said: "There are serious concerns about the future of the orders at the yard and it's important that the Conservative government gives an absolute commitment to them.

"The Tories need to end the feast and famine of orders so the workforce can be maintained at a steady level."

Scottish Green co-convener Patrick Harvie also urged the UK Government to clarify the contracts but he also expressed that the yards only relying on MoD contracts "is not the way forward".

Harvie said: "It's clear that the UK Government must urgently clarify the timescales involved for the upcoming work. But rather than using this situation to repeat the claims and counter-claims we heard during the independence referendum, all parties should be focusing on developing a longer-term plan for jobs on the Clyde.

"We have an opportunity to harness the skills of the workforce, diversify the region's economy and provide real stability and security for families and communities. Pinning all hopes on the idea of everlasting MoD contracts isn't a responsible way forward.

"The Scottish Greens have shown with our Jobs in the New Economy report that opportunities for lasting, well-paid employment exist but we need a bolder Holyrood to prioritise the investment required."

The concerns raised by opposition politicians were however rubbished by the secretary of state for Scotland, David Mundell. He insisted that the UK Government is "absolutely committed" to ship building in Scotland.

Mundell said: "The government is absolutely committed to ship building on the Clyde. We are committed to building the type 26 frigates there and we are committed to these offshore patrol vessels that we announced recently.

"Part of the reason for that announcement was to ensure that there was a transition from the carrier work into the type-26 work. We have made a number of announcements in relation to the type 26 and shipbuilding on the Clyde and we stand by all of them.

"It is obviously for BAE Systems to manage the transition between the carrier work, between the offshore patrol vessels and into the type 26 work and we want to obviously coordinate closely with them to ensure that they aren't job losses.

"What we want to see is a strong future for shipbuilding on the Clyde that was secured last year when people voted to remain in the United Kingdom which means that Royal Navy contracts come to the Clyde. If Scotland had left the United Kingdom those contracts wouldn't have come at all".

When Mundell was asked if he could ensure that there would no job losses he said that it was for "BAE Systems ultimately to manage the transition in their work load" however the UK Government wants to work with the firm "to ensure that there are not job losses on the Clyde".