A new agreement critical to Britain's Trident nuclear weapons system, was signed the other day by British and US officials.

Whitehall was silent. We had to rely on the White House, and a message from Barack Obama to the US Congress, to tell us that the 1958 UK-US Mutual Defence Agreement (MDA) had been updated.

A new amendment to the treaty will last for 10 years. Obama told Congress it will "permit the transfer between the United States and the United Kingdom of classified information concerning atomic weapons; nuclear technology and controlled nuclear information; material and equipment for the development of defense plans; training of personnel; evaluation of potential enemy capability; development of delivery systems; and the research, development, and design of military reactors."

The UK, Obama added, "intends to continue to maintain viable nuclear forces into the foreseeable future." It was in America's interest, to continue to help Britain "in maintaining a credible nuclear deterrent".

There was no word from the Foreign and Commonwealth Office, the Whitehall department responsible updating the UK-US treaty. Parliament, a spokesperson said in response to questions, would be informed "at an appropiate time". MPs would have a 21 day window before the end of the year in which they could debate the issues involved.

However, the content of the new agreement will remain secret. To reveal them, Whitehall officials say, could "assist proliferation" of nuclear weapons.

That is a curious comment given that both the US and UK insist the agreement does not in any way breach their obligations under the nuclear non-proliferation treaty (NPT).

The updated agreement, as I described last month, means that Britain is stepping up its cooperation with the US over the design of nuclear warheads, raising new questions about the independence of the UK deterrent. Increased cooperation with the US on warhead design and the exchange of material crucial in the manufacture and stockpiling of nuclear weapon is vital to the Trident system.

Though the agreement is incorporated in US law, it has no legal status in Britain.

Paul Ingram, executive director of the British American Information Security Council (BASIC) says that though the agreement is an international treaty that requires regular ratification, it has never been debated in the Commons. Questions from MPs were met with "cursory information".

He adds: "With the deepening of technical collaboration that shapes the procurement decisions here in London over nuclear weapons programmes, in a manner that stretches or breaks Article 1 of the NPT, it is high time we took this relationship and its consequences for international security seriously."

Article 1 of the NPT states: "Each nuclear-weapon state party to the treaty undertakes not to transfer to any recipient whatsoever nuclear weapons or other nuclear explosive devices or control over such weapons or explosive devices directly, or indirectly..."

A briefing paper drawn up for ministers and Ministry of Defence officials argues that actual physical "movements under the MDA do not involve nuclear weapons or devices".

Therefore, the agreement does not contravene the letter of the NPT.

There is another, even broader, issue which the government does not want to open up for debate.

The joint Commons/Lords committee on the National Security Strategy has chided the government for not taking such an important issue seriously enough.

"I am afraid that there is little sign that the government has really taken on board the need to put some serious effort into planning for the next National Security Strategy" said former Labour foreign secretary, and chair of the committee, Margaret Beckett.

She added: "I fear that — just as in 2010 — the next NSS will be cobbled together at the last minute after the election, as an adjunct to the spending review, rather than guiding the incoming government's decisions on spending priorities.

"In the run up to the election it is inevitable that the political parties will be focusing on their different policy priorities, but — when it comes to national security — there are strong arguments for identifying where there is common cause."

Referring to government claims, the committee described as "wholly unrealistic" the assumption that there will be "no shrinkage in the UK's influence" despite the rise of Asian powers and Britain's relative economic decline. "Any national security strategy based on this is wishful thinking rather than credible strategy," the committee said earlier this year.

The Commons/Lords committee wants to consult outside experts and is inviting views from the public.