Ben Wallace is under pressure to rethink his failure to back Britain's shipbuilding industry in light of Russian aggression in Ukraine.
The GMB Union warned Mr Wallace the "threat of war is not abstract" - adding the UK's shipbuilding and ship repair sectors "are essential to our national security."
It comes after the MoD told the Defence Select Committee that it will "not provide a guaranteed pipeline of work for UK yards" when the refreshed Shipbuilding Strategy is published.
In a letter to Mr Wallace, General Secretary Gary Smith urged the Secretary of State to "think again and overturn your department’s damaging policy on this matter."
Mr Smith said: "The timing of the MoD’s refusal to back UK shipbuilding could hardly be worse.
"Defence jobs are being slashed across the board while Russian vessels bombard the shores of a sovereign European nation.
"The threat of war is not abstract. We are an island nation - our shipbuilding and ship repair sectors are essential to our national security.
"The war against Ukraine removes any doubt that we must look to our own defences."
An MoD spokesman said: “The forthcoming National Shipbuilding Strategy refresh will work to increase exports, access to finance and productivity, enabling more UK yards to win commercial and export work."
