The UK kept quiet about a loophole allowing the US to continue storing banned cluster bombs on its territory, a leaked US diplomatic cable suggests.
A senior Foreign Office official is quoted in the message, sent in May 2009 and published by the Wikileaks website.
It suggested not formally agreeing an exception for "specific missions" until after the UK Parliament ratified the ban - thus avoiding debate among MPs.
The Foreign Office insists it never deliberately failed to inform MPs.
Dropped from the air or fired from the ground, cluster munitions release small bomblets over a wide area. Critics say they have a devastating humanitarian impact - most victims are civilians; a third are children.
Britain was among more than 90 countries which signed the Convention on Cluster Munitions (CCM) in December 2008. The treaty bans the use of cluster bombs and prohibits signatories from assisting other countries to use, stockpile or transfer them.
The then prime minister, Gordon Brown, hailed the treaty as a "major breakthrough".
But the US - along with other major military powers such as Russia, China, India and Pakistan - was not a signatory. And that clearly put the UK in an awkward position with a key ally.
Britain leases the Indian Ocean island of Diego Garcia to the US, where the Americans have a major base in which cluster bombs are stored. Britain's public stance is that the US military has until 2013 to remove them.
In April this year, the then minister for international defence and security, Baroness Taylor of Bolton, assured Parliament: "I can confirm that the US has identified its cluster munitions on UK territory as exceeding its worldwide operational planning requirements. Therefore, these cluster munitions will be removed from sites in the UK in 2010 and from all UK territories by 2013."
But one of the cables released by the whistle-blowing website, Wikileaks, and reported by the Guardian shows the Foreign Office suggested a loophole to allow the US to keep cluster bombs on British soil should be kept from Parliament.
The cable reveals that the UK offered the Americans "temporary storage exception for specific missions". No details are given, but it is clear that this was something the government was keen to keep quiet.
The cable quotes a senior Foreign Office official as noting: "It would be better for the USG [US government] and HMG [Her Majesty's Government] not to reach final agreement on this temporary agreement understanding until after the CCM ratification process is completed in Parliament, so that they can tell parliamentarians that they have requested the USG to remove its cluster munitions by 2013, without complicating/muddying the debate by having to indicate that this request is open to exceptions."
The cable also reveals that most of the US cluster munitions are being stored on US vessels off Diego Garcia, apparently to circumvent the ban on the weapons remaining on British soil after 2013.
It states that the head of the Foreign Office's Security Policy Group, Nicolas Pickard, "reconfirmed that off-shore storage on US ships would still be permitted".
Responding to the leaked cable, Thomas Nash from the campaign group, Cluster Munition Coalition, told the BBC: "The UK has banned cluster bombs entirely and is bound never to assist with use of cluster munitions by any country ever again. Along with 107 other countries, the UK condemned the use of cluster bombs in the final declaration of the first meeting of state parties to the cluster bomb ban in Lao PDR [People's Democratic Republic] last month.
"The cable also raises concern about respect for Parliament and for democratic practices on the part of the UK government. The so-called exceptions in the UK law should never be used and cluster bombs should never be anywhere near Diego Garcia again," he added.
"If any 'authorisations' are contemplated by the secretary of state, they should be subject to strict parliamentary scrutiny."
A Foreign Office spokesman responded to the publication of the cable by saying: "We reject any allegation that the FCO deliberately misled or failed in our obligation to inform Parliament."