Opposition parties will today renew their assault on the Government over the arms to Africa affair amid signs that the British mercenaries involved will not be prosecuted.

The Conservatives and Liberal Democrats will use a Commons debate to try again to find out exactly how much the Foreign Office knew about Sandline International supplying weapons to the west African state, allegedly in breach of a United Nations embargo.

The Government will also be put on the spot over reports yesterday that ''military consultants'' Sandline, headed by Bosnian war veteran Colonel Tim Spicer, are to escape criminal charges.

A progress report of an investigation by Customs and Excise, currently being studied by Attorney General John Morris, is understood to recommend that prosecutions would not stand up in court due to ''insufficient evidence''. There are also fears in Whitehall that criminal charges could be embarrassing for the authorities.

Mr Morris is due to decide within days what action, if any, to take although he is widely expected not to initiate any prosecutions. Contacts between the Foreign Office and Sandline are being blamed for the lack of charges.

Controversy surrounds whether the mercenaries breached a UN order banning the export of arms to Sierra Leone when they shipped 30 tons of guns last autumn to help the then-exiled deposed democratic President Kabbah regain his position after a coup. Sandline says that the embargo only applied to arms supplies to the military junta, which was ousted, rather than to the president.

Ministers have been accused of giving Sandline ''a nod and a wink'' to illegally supply weapons to help achieve the Government's aim of restoring President Kabbah.

In the Commons today embattled Foreign Secretary Robin Cook and his deputy Tony Lloyd, who have offered unconvincing explanations so far of the Foreign Office's behaviour, will be pressed to answer ''key questions'' about their department's role in restoring democratic rule in Sierra Leone.

A Tory motion deplores ''the muddle of government policy towards Sierra Leone'' and denounces ''contradictory statements'' made by Tony Blair, Mr Cook and the top civil servant at the Foreign Office. It demands to know what ''knowledge and involvement'' Ministers, officials and the intelligence services had of Sandline's activities.

Shadow foreign secretary Michael Howard yesterday renewed his call for a full public inquiry, headed by a judge, to investigate the affair. ''I want the full truth and I want it as soon as possible'', he said.

Recalling the Foreign Secretary's earlier pledge to MPs to be open about the arms affair, Mr Howard said all the available evidence had to come out ''so that everybody can see exactly what went on, exactly who knew what, and exactly who said what to whom''.

A Liberal Democrat motion also seeks full information about the authorities' links with ex-British Army forces in Bosnia spokesman Colonel Spicer's outfit, and endorses the call for an independent inquiry.

Party foreign affairs spokesman Menzies Campbell said it was vital to know why Ministers were apparently not kept fully informed.

The Prime Minister, who last week dismissed the sanctions-bustings claims as merely ''hoo-ha'', yesterday backed Mr Cook's handling of the arms saga. Restoring President Kabbah to power was ''a good thing, not a bad thing'', he said.

He insisted the Government's dealings with Sierra Leone were a good example of its ''ethical'' foreign policy. The Foreign Secretary had led the restoration ''from a brutal military coup d'etat of a democratically elected regime''. It was an example of ''high ground'' policy, said Mr Blair.

A spokeswoman for the Attorney General said he was still considering the Customs report.