LONDON (REUTERS) - The opposition Labour party would look at cutting the fleet of submarines carrying Britain's Trident nuclear deterrent to three vessels from four if it wins a national election on May 7, the party's finance spokesman Ed Balls said on Sunday.

Replacing the vessels carrying the Trident missiles - four ageing Vanguard-class submarines - is expected to cost 20 billion pounds (S$41 billion), with a final decision on the renewal due to be taken in 2016.

The Scottish National Party, who could hold the balance of power after the election, oppose the renewal but this month dropped a demand Labour must also do so in return for their support.

Balls said a Labour government would hold a defence review after the election and, while the party was committed to renewing Trident and maintaining a continuous at-sea deterrent, this did "not necessarily" mean replacing all four submarines.

"If it turns out the only way strategically to deliver'continuous at-sea' is four boats, then absolutely we will have four boats, but my job is to ask those questions, and therefore we are going to challenge to see if there is any way you could do it with three rather than four," Balls told BBC Radio.

Prime Minister David Cameron's Conservatives, neck-and-neck with Labour in many polls, favour a like-for-like replacement, while their junior coalition partners, the Liberal Democrats, believe the fleet should be scaled down.