By Phil ScullionFollow @PhilScullion
Caroline Lucas has uncovered "illegal promotion" of banned cluster bombs at the London arms fair.
In a letter to business secretary Vince Cable the Green party leader raised the issue of the illegal weaponry and questioned the government's control over the event.
Ms Lucas said: "I was able to find illegal advertising materials on the basis of one short visit to the exhibition and with very few resources at my disposal. There is no telling what other breaches are occurring and might be uncovered with further research.
"It should not be left to MPs and campaigners to police illegal promotion of banned arms on British soil."
Ms Lucas found the cluster bomb promotions at two stands linked to Pakistan.
Cluster bombs are only permitted to be promoted if they comply with the provisions of the Oslo Accord which has 110 states as signatories and was first signed by the UK in December 2008.
The Defence & Security Equipment International (DSEi) exhibition has already proved controversial with many criticising the number of authoritarian regimes represented, such as Saudi Arabia and Bahrain.
Fourteen authoritarian regimes are represented at the DSEi and five of those are named by the Foreign Office as among the world's worst human rights offenders, Ms Lucas added.
Liam Fox has defended the arms fair, saying that arms exports are a crucial feature of the economy, but Ms Lucas criticised the defence secretary's stance.
She said there is an "inherent conflict between the government's promotion of military exports and its stated desire to help protect human rights overseas".