Britain has committed itself to buying a new generation of nuclear warheads to replace Trident, which will be based on US technology. The decision was revealed by Pentagon officials who disclosed it before an official announcement has been made by the government.

The revelation has dismayed MPs and experts who question why they have learned of the move – which will cost the UK billions of pounds – only after the decision has apparently been made. It has also raised questions about the UK’s commitment to staunching nuclear proliferation and the country’s reliance on the US for a central plank of its defence strategy.

Earlier this month, Pentagon officials confirmed that its proposed W93 sea-launched warhead, the nuclear tip of the next generation of submarine-launched ballistic missiles, would share technology with the UK’s next nuclear weapon, implying that a decision had been taken between the two countries to work on the programme.

In public, the UK has not confirmed whether it intends to commission a new nuclear warhead. The Ministry of Defence’s annual update to parliament, published just before Christmas, says only: “Work also continues to develop the evidence to support a government decision when replacing the warhead.”

But last week Admiral Charles Richard, commander of the US strategic command, told the Senate defence committee that there was a requirement for a new warhead, which would be called the W93 or Mk7. Richard said: “This effort will also support a parallel replacement warhead programme in the United Kingdom, whose nuclear deterrent plays an absolutely vital role in Nato’s overall defence posture.”

Ed Davey, acting leader of the Liberal Democrats, said: “It is totally unacceptable that the government seems to have given the green light to the development of new nuclear weapon technologies with zero consultation and zero scrutiny. Britain under Johnson increasingly looks like putty in Trump’s hands. That Britain’s major defence decisions are being debated in the United States, but not in the UK, is a scandal. Under Johnson, it seems that where Trump leads, we must follow.”

Alan Shaffer, Pentagon deputy under-secretary of defense for acquisition and sustainment, also made reference to the new UK programme in a briefing session at the annual nuclear deterrence summit, in Alexandria, Virginia. “I think it’s wonderful that the UK is working on a new warhead at the same time, and I think we will have discussions and be able to share technologies,” Shaffer said.

David Cullen, director of pressure group the Nuclear Information Service, said: “The UK’s reliance on US knowledge and assistance for their nuclear weapons programme means they will find it almost impossible to diverge from any development path the US decides to take. “We are legally bound to take steps towards disarmament under the nuclear non-proliferation treaty, but this would take us in the opposite direction.”

It is understood that the US had agreed with the UK not to make any announcement while parliament was in recess. However, US defence officials were apparently oblivious to the agreement and confirmed the programme’s existence – to the embarrassment of the UK government.

Hans Kristensen, director of the nuclear information project at the Federation of American Scientists, said the development of the new warhead posed significant geopolitical problems. “Britain and the US have come a long away from being leaders in reducing the role of nuclear weapons and contemplating the possible road toward potential disarmament to re-embracing nuclear weapons for the long haul. They are obviously not alone in this, with Russia, China and France doing their own work. So, overall, this is a serious challenge for the international non-proliferation regime,” he said.

Tom Plant, director of proliferation and nuclear policy at the independent security thinktank, Rusi, said the lack of debate about the new weapon was a concern. “There’s been a presumption from those in opposition and analysts like myself that it should come to parliament in some way, like the 2016 vote on Trident. I suspect that the MoD’s position is that they don’t want it to. What the programme doesn’t need from their perspective is lots of scrutiny. But if there’s going to be a decision it should absolutely come to parliament.”

The MoD said: “As previously stated in the 2015 defence review, we can confirm that we are working towards replacing the warhead. We have a strong defence relationship with the US and will continue to remain compatible with the US Trident missile. An announcement about the UK’s replacement warhead programme will be made in due course.”