Britain is trying to boost defence equipment sales to Libya by sending a Royal Navy warship to Tripoli to act as a floating shop window for security firms, amid concern in Whitehall that France and Italy are cashing in on the fall of Muammar Gaddafi.

But the trip, planned for April, has raised concerns among Libyan politicians and arms control campaigners who have demanded to know which companies will be on board and what kind of equipment they will be attempting to sell.

So far, UK Trade and Investment, the government agency organising the arms fair, has refused to disclose the businesses likely to be exhibiting, saying this would give European competitors an advantage.

UKTI said no weapons would be offered for sale and the Libyans would only be shown specialist equipment to help with port security and maintenance, as well as ribbed inflatable boats for patrolling the harbour, and uniforms.

Yet prominent Libyans have raised fears that the race to win defence contracts could lead to equipment getting into the wrong hands in a country where the government has a tenuous grip on security.

"I can't see the point of having this kind of exhibition in Libya now," said Hassan el-Amin, an independent member of congress, who lived in exile in the UK for 28 years and who is chair of the congress communications committee. "One of our problems is that arms are everywhere. I can't see any point in an arms exhibition right now."

The fair appears to be part of the UK government's defence engagement strategy, which is designed to foster relations in areas where the UK has security – and business – interests.

The Ministry of Defence confirmed that a Royal Navy frigate would make a port call to Tripoli in the spring, but said it could not give further details for security reasons. However, the UKTI website said it was seeking British defence firms to take part in a "defence and security industry day" in Tripoli in April.

The agency described the event as "an opportunity for UK defence and security to promote equipment and services to the Libyan navy on board a Royal Navy vessel in Tripoli." It would "attract key senior military personnel from the Libyan government".

Registration for the event closed on 12 February, but UKTI – which says it "provides the essential government to government dimension to help the UK defence and security industry" – said the list of exhibitors was not complete.

With a UN arms embargo in place and the Foreign Office regarding Libya as a "country of concern" over human rights abuses, Britain is restricted in what it can sell. Sir John Stanley, chair of the committees on arms export controls, said he expected ministers to adhere to their own criteria when considering arms export licences.

William Hague, the foreign secretary, has said that if there is a "clear risk that [an] export might provoke or prolong regional or internal conflicts, or which might be used to facilitate internal repression", then a licence should be refused.

Stanley said: "The policy is not qualified. The policy is clear and we expect the government to adhere to it."

His committees will be able to review licences granted. "The defence companies can show what they like, but they still have to get valid export licences before anything leaves the UK," Stanley added.

The Campaign Against the Arms Trade said it had concerns about the trade mission and use of a warship as a platform for the defence trade. "We'd rather it didn't happen at all. But if it is going to happen, then UKTI needs to be more transparent about who is going and what they are attempting to sell," said a spokesman.

There is already an undeclared arms race among European defence contractors to re-equip Libya's armed forces, which were defeated in the 2011 Nato-backed Arab spring uprising. Last year Chris Baker, operations director for UKTI's defence arm, said Britain regarded Libya as a "priority country" for future exports. Speaking at the Farnborough air show, he said the UKTI was looking at Libya's border and maritime security and "at rebuilding their defence infrastructure, getting their air force back on its feet from scratch".

This month Italy handed over 20 Puma armoured cars to Libya's defence ministry, and in January the French company Sillinger sold Libya 50 rigid inflatable boats.

Mohammed al-Bargati, the Libyan defence minister, said Libya would give Italy "priority status for new armaments", although the policy is complicated by the continuing UN arms embargo on Libya.

Weapons proliferation remains a headache for Libya, with Libyan weapons reportedly fuelling the Mali insurrection. The former US secretary of state Hillary Clinton said she had no doubt Libyan weapons were used in the attack on the In Amenas gas plant in Algeria, which left 39 hostages dead.

"The Pandora's box of weapons coming out of these countries in the Middle East and north Africa is the source of one of our biggest threats," she said. "There is no doubt that the Algerian terrorists had weapons from Libya."

Bilal Bettamer, a Benghazi lawyer and democracy activist, said: "It's not a secret that Libyan weapons are going everywhere, you might find them being used against British soldiers. I hope these weapons go to the official army, the trusted people."

A previous UKTI DSO mission to Libya in April 2012 was attended by BAE Systems, CAE, GD (UK), KBR, NATS, 3SDL, Selex Galileo, and Surrey Satellite Technology, according to an answer to a parliamentary question by MP Ann Clwyd on 24 May.

Britain has taken a lead in offering training for Libya's army and police, and the EU is providing assistance as Libya seeks to monitor its porous borders, amid concerns that jihadist groups can cross at will.

The Guardian reported in March 2011 that UK companies gained arms export licences for Gaddafi's forces worth €72.2m (£62m) following the lifting of the international arms embargo in October 2004. When Libya's revolution broke out, the government revoked 158 export licences for UK companies to Libya.

In an article in the Guardian on 17 September 2012, the MPs Douglas Alexander and Jim Murphy called for tighter regulation over arms sales to Libya, warning: "If the Arab spring has shown us anything, it is that perceived past stability is no guarantee against future volatility".