Theresa May will face MPs in the House of Commons for the first time as prime minister this afternoon (18 July 2016), as the Conservative premier leads a debate on the renewal of the UK's nuclear deterrent, Trident. May is expected to declare that it would be an irresponsibility if Britain ditched the system.

The programme, a continuous at sea deterrent using Vanguard-class submarines, cost the government £18.35bn ($24.31bn) to establish, according to the House of Commons Library.

Its renewal, which will include the design and manufacturing of new four new submarines, is estimated to total £31bn ($41.1bn, €37.2bn) over the system's 20-year life.

The government has also planned a £10bn contingency to be set aside.

Labour is split three ways over the system, with Jeremy Corbyn expected to vote against its renewal today.

His shadow business and foreign secretary, Clive Lewis and Emily Thornberry, have urged Labour MPs to abstain on the vote, and some backbenchers such as Barrow and Furness' John Woodcock backing the programme.

Unite and GMB, both Labour-supporting trade unions, also support Trident's renewal, with thousands of their members' jobs at risk in the defence industry if the programme is scrapped.

But Lewis and Thornberry, two top allies of Corbyn, have claimed the vote is a "contemptible trick" from the Conservatives. "Labour should not play this game. We should treat this government and this vote with the contempt they deserve," the pair wrote in The Guardian.

"Moreover, there are clear principled and practical reasons why Labour MPs should refuse to vote with the government on Monday. They propose an open-ended commitment to maintain Britain's current nuclear capability "for as long as the global security situation demands".

"Such a vague, indefinite commitment precludes any possibility of Britain ever stepping down the nuclear ladder and contributing to global multilateral disarmament."

Labour's current policy is to back the renewal of Trident, a stance which faced changes because of Thornberry's defence policy review. The findings of the probe were expected a week after the EU referendum, but the Brexit result has reportedly delayed the publication of the report.

What the Trident nuclear deterrent is and why it matters to UK security?

Trident operates a continuous at-sea nuclear deterrent. That means one of the programme's four nuclear submarines, which are based in the Faslane area of Scotland and operated by the Royal Navy, is always on patrol.

These Vanguard-class submarines are around 491ft in length, or more than twice the size of two Boeing 747s, and powered by steam. A nuclear reactor inside the underwater vessels boils sea water, the steam from which is then used to propel them.

The four submarines – HMS Vanguard, HMS Vengeance, HMS Victorious and HMS Vigilant – are capable of carrying 16 Trident II D5 ballistic missiles, produced by the American arms manufacturer Lockheed Martin, each armed with up to eight nuclear warheads. As it stands, each submarine only carries three Trident missiles.

Their power and precision is stunning. Each missile is 44ft long, 83 inches in diameter, capable of exceeding speeds of more than 13,000mph, and can hit targets up to 7,000 miles away, accurate to within a few feet. Read more here.