The British government has just published amendments updating a treaty that goes to the heart of the UK's special relationship with the US.

They relate to the Mutual Defence Agreement (MDA) first signed in 1958, which, according to the government, enables the UK and the US "nuclear warhead communities to collaborate on all aspects of nuclear deterrence including nuclear warhead design and manufacture".

One amendment refers to potential threats from "state or non-state actors". But the amendments are for the most part arcane and their significance cannot be understood in the absence of information which is kept secret.

The MDA does not have to be debated or voted on in parliament, as I have remarked before. Though the agreement is incorporated in US law, it has no legal status in Britain.

Yet the matters covered by the treaty, which is renewed only at 10 year intervals, are hugely important. Successive British governments have made clear a proper debate on the issues involved would not be welcome.

"A debate on the renewal of the MDA would be used by some as an opportunity to raise wider questions concerning the possible renewal of the nuclear deterrent...and our obligations under the nuclear non-proliferation treaty," notes an internal MoD paper, dated 2004. The paper was released only earlier this year through a freedom of information act request by the independent Nuclear Information Service.

The Labour MP Jeremy Corbyn has applied for a backbench business debate on this latest renewal of the MDA.

Yet it should be given much greater priority, and subjected at least to scrutiny by the Commons crossparty defence committee.

In an "explanatory memorandum for MPs, the government says the MDA "does not (a word it underlines) provide for the transfer of nuclear weapons...or control over such weapons or nuclear devices."

It suggests that, as far as the UK is concerned the advantages are purely economic - the agreement allows the UK "to significantly reduce costs while maintaining an operationally independent deterrent".

Yet a report this summer by the all-party Trident Commission set up by the thinktank BASIC (the British American Security Information Council)

suggests that Britain's nuclear weapons system is far from independent.

Without the cooperation of the US, the life expectancy of the UK's nuclear capability could be measured in months, it said.

A document obtained by the Nuclear Information Service refers to "enhanced collaboration" between the two countries on "nuclear explosive package design and certification".

Another document describes the MDA as an agreement that enables the British and American "nuclear warhead communities to collaborate on all aspects of nuclear deterrence including nuclear warhead design and manufacture".

Paul Ingram, BASIC's executive director, says: "In governing exceptional nuclear weapon collaboration between the US and UK that has contested legal basis, the MDA is the manifestation of the deep political, cultural and philosophical relationship between the two states."

He adds: "How can it possibly be effective to criticise North Korea for allegedly supplying nuclear and missile technology to states like Syria and Iran when we trade between ourselves technologies directly relevant to constructing nuclear warheads, missiles and submarines?"

Kate Hudson, general secretary of the Campaign for Nuclear Disarmament (CND) says the UK-US agreement flew in the face Britain's commitments as a signatory to the nuclear Non-Proliferation Treaty.

"It is appalling that David Cameron is signing secretive nuclear deals behind Parliament's back. In no other area of government would such a sinister sidestepping of democratic process be tolerated."

She has a point.