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, according to documents disclosed after a freedom of information request.

Increased cooperation on warhead design and the exchange of material crucial in the manufacture and stockpiling of nuclear weapons will be sealed in a pact being drawn up by senior officials from the two countries.

The pact, renewing the 1958 mutual defence agreement (MDA) between the UK and US, is expected to be signed in a discreet ceremony in Washington in the next few weeks. It does not have to be debated or voted on in parliament. Though the agreement is incorporated in US law, it has no legal status in Britain.

A document prepared for a visit by a senior American nuclear official to the Aldermaston atomic weapons establishment (AWE) refers to "enhanced collaboration" on "nuclear explosive package design and certification", on "maintenance of existing stockpiles", and the "possible development of safer, more secure, warheads".

The partially censored document refers to a letter Tony Blair wrote to George Bush in 2006 asking for US help in maintaining Britain's "nuclear delivery system" and the white paper of the same year, which gave the green light for replacing the existing fleet of Trident nuclear missile submarines.

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

A briefing paper drawn up for ministers and Ministry of Defence officials argues that physical "movements under the MDA do not involve nuclear weapons or devices" and therefore the agreement does not contravene the letter of the nuclear non-proliferation treaty (NPT).

Most of the documents now released were drawn up at the time of the last renewal of the MDA in 2004. They make it clear Whitehall did not welcome a debate in parliament about the mutual defence pact. "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, the future of AWE and our obligations under the nuclear non-proliferation treaty," an internal MoD paper says.

It adds that having refused requests from individual MPs to allocate time for a debate on MDA renewal, the government might nevertheless have to acceed if Commons committees demand greater parliamentary and public scrutiny of the issues involved.

It says that "highly complex and sensitive issues will need to be opened to parliamentary scrutiny in due course".

Peter Burt of Nuclear Information Service, who obtained the papers, told the Guardian: "The UK and US are setting a dreadful example to the rest of the world by renewing the MDA, and are seriously undermining the credibility of international efforts to prevent the proliferation of nuclear weapons."

He added: "If Iran and North Korea had signed a similar agreement for the transfer of nuclear weapons technology, the UK and US would be branding them pariah nations and screaming for the toughest of international sanctions to be imposed."

Renewing the MDA showed the "worst kind of two-faced hypocrisy" and demonstrated that neither nation was serious about meeting its legal obligations under the NPT, Burt said.

The MoD said the agreement would be renewed by the end of the year.