The Conservatives have claimed that a Labour government could put the future of the Trident nuclear deterrent at risk.
Defence Secretary Michael Fallon accused Labour of using Trident as a “bargaining chip” in their negotiations with the SNP, who want to scrap the system.
Labour leader Ed Miliband hit back at Fallon, saying that he “demeaned” himself and his office by making political accusations.
The Conservative manifesto will have a commitment to build and deploy four new nuclear submarines to replace the ageing Trident system that will need to be replaced within the next decade.
Labour have also committed to renewing Trident, but have said that they have not yet decided whether the new system will require three or four submarines to maintain a continuous and global deterrent.
The Liberal Democrats have said that they favour a three submarine system, with the current system designed in the 1980s under the conditions of the Cold War era.
The government estimates that replacing Trident would cost around £20bn, but ordering fewer submarines could reduce the final bill by nearly a quarter.