´óÏó´«Ã½

A decision on whether to replace the UK's Trident nuclear-armed submarines is due to be made next year, although the current systems will not have to be taken out of service until 2028.

The Ministry of Defence replacing Trident with four new submarines would cost about £20bn at 2006 prices, which is about £25bn at current prices, but you may have seen much higher numbers circulating.

The uses the figure of £100bn, which comes from the Campaign for Nuclear Disarmament (CND).

That figure includes the cost of maintaining the system, which is put at between 5% and 6% of the UK defence budget, which in turn is between £1.7bn and £2.1bn a year.

If you multiply that up over an estimated 30-year life of the system, that comes to between £51bn and £62bn (although the current system is likely to be used for closer to 40 years).

The CND also makes an estimate of £13bn for decommissioning and you can see how it gets to £100bn.

These are not precise figures, for two reasons. The first is that anything that tries to predict costs over a 30-year period includes a great deal of uncertainty.

The second is that the MoD does not have a great record of predicting the costs of projects. Margaret Hodge, who chairs the Commons Public Accounts Committee, in 2013 that "between 2000 and 2012 the cost of its 69 largest projects ballooned by £11bn. Independent analysis in 2009 found that final project costs were typically 40% higher than the ministry's initial forecasts."

A review has suggested cutting the number of missiles and warheads carried by each submarine to save money, although it is an amount within the range of uncertainty surrounding these figures. The Liberal Democrats have suggested reducing the number of submarines from four to three, which they say would save £4bn a year.

What's the truth behind the politicians' claims on the campaign trail? Our experts investigate the facts, and wider stories, behind the soundbites.

Read latest updates or follow us on Twitter