Scottish National Party (SNP) leader Nicola Sturgeon vowed her party will not enter into an official coalition with Labour unless it ditches its support of Britain's nuclear missile submarine fleet, in the event of a hung parliament in the upcoming knife-edge general election in May.

Prime Minister David Cameron's Conservative Party promised to renew the country's ageing fleet if it wins the 7 May poll, hoping the issue could prove an electoral problem for its main rival, Labour.

Replacing the vessels carrying the Trident missiles – four Vanguard-class submarines – is expected to cost £20bn ($29.7bn), with a final decision on the renewal due to be taken in 2016.

Opponents argue replacing Trident could cost up to £100bn and Britain should consider cheaper alternatives.

With neither Labour nor the Conservatives forecast to win the election outright, the most likely route to power for Ed Miliband's party is a deal with the SNP, who vocally oppose any renewal of the fleet.

But Sturgeon said renewal of Trident is a "red line" issue for her party. A twist to the issue is the nuclear missile submarines are based in a Scottish loch.

She said: "The SNP have made it very clear that Trident is a fundamental issue for the SNP so we would never be in any formal deal with a Labour government that is going to renew Trident and we would never vote for the renewal of Trident or for anything that facilitated the renewal of Trident.

"It's one example of where we would like to build progressive alliances in the House of Commons so we can ensure £100bn is spent not on weapons of mass destruction that can never be used but on building a better future for our children, that money is far, far better invested in our health service and in our education system and in better childcare for our youngest."

Miliband reiterated his party would maintain a continuous-at-sea nuclear presence, although it has mooted the idea that three submarines could fulfil the same role.