State prosecutors were yesterday accused of “abject failure” to ensure two defence companies answer allegations of marketing torture equipment at the world’s largest arms fair in London after a private case against the firms collapsed.

Campaigners earlier this year won an unprecedented ruling to bring a private prosecution against French military equipment firm Magforce International and a similar Chinese company, Tianjin Myway International. Both companies were ejected from the Defence and Security Equipment International (DSEI) exhibition in Docklands last September after brochures, seen by The Independent, were found on their stalls apparently offering for sale items, including leg irons and electric stun batons, which cannot be marketed or exported from the United Kingdom.

But the case has now been discontinued after campaigners said the CPS made it clear at a meeting with lawyers bringing the case last week that it could no longer proceed for legal reasons.

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Campaigners had long called for HM Revenue & Customs, which investigates breaches of export rules, and the CPS to mount a public prosecution but said they were forced to launch their own proceedings in February after receiving no response.

Under rules which mean any prosecution for breach of export regulations must be brought within six months of the claimed offence, proceedings can now no longer be brought in any form against either of the two firms.

It is understood campaigners are now considering a formal complaint against the CPS and the HMRC as well as a possible judicial review.

Raj Chada, of law firm Hodge Jones & Allen, who was bringing the case, said: “Following the meeting with the CPS, it is clear that our client cannot continue their private prosecution.

The state authorities as a whole have failed to properly investigate or build a case in this matter. We will be considering our next step with regard to the abject failure of the state to hold companies that allegedly break the law to account.”

The private prosecution had alleged that items being marketed by the two companies came under “Category A” of Britain’s Export Control Order 2008, which describes items such as fetters and stun weapons as “undesirable” and effectively bans their sale or promotion because of their potential for use in torture.

The Independent witnessed the eviction of the firms from the vast DSEI exhibition, which bills itself as “the world-leading defence and security event” and is attended by all of the world’s largest weaponry manufacturers as well as arms-buying delegations from across the world.

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Following the ejection, Clarion Events, the organiser of DSEI, said it had passed information about the incident to HMRC. A district judge authorised the private prosecution in February and singled out the apparent failure of the CPS to answer requests to mount its own case as a reason for allowing the unusual case to go ahead.

The CPS has said the case was never passed to its lawyers by the HMRC for a decision on whether to bring proceedings.

In a statement, the campaign group Arms Dealers on Trial, said:

“We are appalled that the case has been discontinued. There was a strong prima facie case against the arms companies for having promoted torture equipment for sale at the arms fair.

"Neither the CPS, police or HMRC took steps to make the arms companies accountable. There appears to be no effective legal mechanism to prevent the promotion for sale of illegal torture weapons.”

Representatives of both Magforce and Tianjin, which would have faced fines of up to £5,000 on each of three charges of breaching an export control order if convicted, insisted the items in their brochures had not been available for sale in Britain.

Magforce said it had not thought any of its products were illegal in the UK. Neither company responded to requests to comment yesterday.

The CPS said last night that the private prosecution had not proceeded because it had not obtained the necessary consent from the Director of Public Prosecutions. It also said that in its view the case lacked sufficient evidence to provide a realistic prospect of conviction.

A spokesman said: "Following our meeting with the lawyers representing the private prosecutors, the private prosecutors decided to discontinue the case themselves and took action to do so.”

The HMRC did not respond to a request to comment but has previously declined to discuss any investigation into the DSEI incident.

A spokesman said: "HMRC cannot comment on individual cases. We consider all credible information we receive regarding potential breaches of UK strategic export controls and take action where we find evidence of abuse.”