Downing Street’s integrated review of defence and foreign policy is expected next week to signal a potential increase of the number of Trident nuclear warheads for the first time since the end of the cold war. Whitehall sources indicated that a cap on total warhead numbers – currently set at 180 – is expected to increase, although the exact figure is not yet known, in a move that analysts said was diplomatically provocative. The UK’s stockpile of nuclear weapons peaked at about 500 in the late 1970s, but had been gradually decreasing ever since as the perceived threat from the Soviet Union and now Russia had been assumed to be decreasing. The last strategic defence review, in 2015, committed the UK to “reduce the overall nuclear weapon stockpile to no more than 180 warheads” by the mid 2020s – and reducing the numbers of operationally available warheads to 120. Each warhead is estimated to have an explosive power of 100 kilotons. The atomic bomb dropped on Hiroshima at the end of the second world war was about 15 kilotons. The full reasons for the anticipated move are not yet clear but it comes amid speculation it is designed to help persuade the US to co-fund aspects of a Trident replacement warhead for the the 2030s. Its costs, too, are uncertain. “If this is confirmed, this would be a highly provocative move,” said David Cullen, the director of the Nuclear Information Service. “The UK has repeatedly pointed to its reducing warhead stockpile as evidence that it is fulfilling its legal duties under the nuclear non-proliferation treaty. “If they are tearing up decades of progress in reducing numbers, it will be a slap in the face to the 190 other members of the treaty, and will be regarded as a shocking breach of faith.” Britain has operated its own nuclear weapons since the 1950s but for the past 60 years, following an agreement between the then prime minister, Harold Macmillan, and the then US president, John F Kennedy, the UK has been heavily dependent on US technology. Trident missiles are deployed in four submarines, one of which is continuously at sea to make sure it can strike back in the event of an unprovoked nuclear attack. It relies on an existing US W76 warhead, based on a 1970s design, called Holbrook. However, the W76 is ageing, and the US has proposed developing a more powerful replacement, called the W93. The UK is particularly keen for the US to start work on the W93 and last summer the defence secretary, Ben Wallace, lobbied Congress for the work to go ahead. British MPs voted to renew Trident in principle in 2016, but the Commons is expected to have to vote on a new warhead at some point. In 2016, the Conservatives almost uniformly backed renewal, the SNP voted against, while Labour was split. The MoD has said developing the next generation of Dreadnought submarines to carry the new warhead would cost £30bn plus a £10bn contingency. But officials have so far refused to say how much the warhead would cost. An MoD spokesperson said: “The UK is committed to maintaining its independent nuclear deterrent, which exists to deter the most extreme threats to our national security and way of life. “Replacing the warhead and building four new Dreadnought class submarines are UK sovereign programmes that will maintain the deterrent into the future. We will not comment on speculation about the integrated review, which will be published on Tuesday.”