ROBIN Cook came out fighting in the arms-to-Africa affair yesterday in a bid finally to kill off suspicions that the Foreign Office had turned a blind eye to sanctions-busting.

The Foreign Secretary revealed that his officials had specifically warned the British mercenaries involved in the saga not to send weapons to Sierra Leone.

They left ''military consultants'' Sandline International in no doubt they would be flouting a United Nations embargo if they supplied guns to help topple the country's dictator.

In a heated Commons debate, Mr Cook angrily defended the Foreign Office against Conservative claims that its handling of the alleged illegal supply of guns to the west African state had made it ''the laughing stock of the world''.

As it was confirmed that Sandline would not be prosecuted for illicit arms trafficking, Mr Cook pledged that a two-man inquiry would establish once and for all if the firm received any encouragement from anyone in officialdom.

In a statement, Customs and Excise announced no criminal charges would be brought despite its barely disguised belief that the London-based mercenaries had indeed broken the law by breaching the UN embargo forbidding the supply of arms to Sierra Leone.

But in a resort to classic Whitehall-speak, Customs - which has spent several months investigating Sandline's actions - hinted that contacts between Foreign Office personnel and the mercenaries were to blame for the decision not to lay charges.

Using deliberately vague language, the Customs statement said: ''Even though offences may have been committed, the particular circumstances leading up to the supply affect the fairness of the case to the extent that any prosecution could well fail and would certainly not be in the public interest.''

A Customs official refused to explain what the crucial ''particular circumstances'' were preceding Sandline's export of 30 tonnes of firearms in February designed to help Sierra Leone's deposed leader, President Ahmad Tejan Kabbah, regain his position.

But they are understood to centre on discussions between Sandline and officials about whether sending weapons to assist the ousted president, rather than the country's brutal military dictatorship, would be illegal.

It is now obvious that those links fatally undermined any chance of a successful prosecution.

In the Commons yesterday, the Foreign Secretary stressed that there had been only one face-to-face meeting between his officials and Sandline. At that meeting, Foreign Office staff had plainly spelled out that despatching arms to Africa would break the law.

According to Mr Cook, at that meeting Sandline said they had heard that someone else was planning to send shipments to the troubled west African state and inquired what the legal position would be.

''The text of the Security Council Resolution was fetched from a neighbouring room and read to the representatives of Sandline,'' Mr Cook told MPs. ''They asked if the reference to (the illegal status of arms deliveries to) Sierra Leone included everyone connected with Sierra Leone and were advised that this was the case.''

Sandline had claimed they thought the embargo did not apply to anyone helping President Kabbah engineer his return to office, which the Government strongly backed. Mr Cook used this revelation, during an often-heated Commons debate, to try and restore his department's battered reputation.

He insisted that Ministers and officials had acted properly at all times. ''There was no failure of policy. Nor was there any failure to act upon intelligence,'' Mr Cook insisted. ''Nor has there been any cover-up. We have acted openly throughout.''

Foreign Office staff in fact prompted the Customs investigation by alerting officers to their suspicions about Sandline. ''That is not the action of people who themselves connived at the breach which they were asking Customs to investigate,'' said Mr Cook.

He announced an ''entirely independent'' inquiry into the affair. It will be conducted by Sir Thomas Legg, a recently retired senior civil servant, and businessman Sir Robin Ibbs, chairman of Lloyds TSB bank and a former adviser to Baroness Thatcher.

They will start work at once investigating two key questions. First, ''whether any department of Government gave approval to breaches of the arms embargo on Sierra Leone''. In addition, they will examine ''what was known by the Government, either by Ministers or officials, of any alleged breach''.

But both the Tories and the Liberal Democrats immediately criticised Sir Thomas's appointment and claimed a High Court judge, rather than a lifelong civil servant, should have been put in charge.

q The Government faces a legal bill of more than #1m after abandoning proceedings against directors of an arms firm involved in the Iraqi supergun affair. In the High Court yesterday, the Department of Trade and Industry was formally given leave to drop disqualification proceedings against the former chairman of Astra Holdings, Gerald James, and three co-directors. Mr James has long maintained he was victimised by the last government after exposing the covert supply of arms by British firms to Iraq.