In the genteel world of clay pigeon and game shooting, the Sportsman Gun Centre is something of a household name as Britain's leading purveyor of hunting rifles and related paraphernalia ranging from silk ties adorned with pheasant motifs, through moleskin breeches and tweed gilets, to full camouflage suits.

Lately however, the Exeter-based company appears to have been branching out into rather more exotic territory: details released by the government under the Freedom of Information Act reveal that it received licences last year to export more than 1,000 assault rifles, combat shotguns, pistols and other weapons to Sri Lanka.

The licences were granted at a time when UK arms exports to the south Asian country have been on the increase. Sri Lanka was the stated destination for military items worth more than £3m in just three months last year. More than £2m of those exports was under the "ML1" label – used by the UK's Export Control Organisation (ECO) to denote small arms and weapons.

Yet, rather than going to the military of a country still classified by the Foreign Office as a "country of concern" for human rights abuses, the weapons sales are an apparent spin-off from a boom area for many British businesses – the protection of shipping from Somali pirates.

It is four years since raiders based on the Somali coast began to terrorise the busy shipping lanes of the Indian Ocean, and in that time the maritime security business has mushroomed on an unprecedented scale. It is now worth £100m a year to British companies according to ADS, a trade organisation for the UK's aerospace, defence, security and space industries.

UK-based maritime security companies account for some 75% of the market, and more than 300 security companies operate in the Indian Ocean region including from Sri Lanka. "We absolutely dominate the market," says Paul Gibson, a former army officer and director of counterterrorism at the Ministry of Defence who now directs the Security in Complex Environments Group (SCEG), which was established by ADS in 2011.

"On the back of Iraq and Afghanistan and our general reputation, shipowners feel that a British security company is going to do the right sort of thing. We were in at the beginning and we have pretty much maintained that reputation, remaining the first choice for many of the big owners. There are an awful lot of ex-Royal Marines in the business."

The arms sales do not only go to Sri Lanka, where private military security companies involved in anti-piracy operations store their munitions and weapons. Further export control statistics show that export licences for £8.5m of specifically military items, together with £36.8m worth of dual-use (civil and military) items, were granted last year for exports to Kenya – another base for maritime security companies (and, incidentally, pirate bosses).

The export records come with a footnote explaining exactly what is being sent to protect the shipping lanes: "Licences … were issued for use in maritime anti-piracy operations – assault rifles, body armour, components for assault rifles, components for pistols, components for rifles, direct view imaging equipment, military helmets, pistols, rifles, small arms ammunition, weapon sights, components for body armour, components for sporting guns, high quantities of sporting guns and combat shotguns."

The increased focus on security at sea appears to be working. To date, no ship with armed guards has been hijacked in the Indian Ocean, a region that saw 189 pirate attacks in 2011 alone. Estimated ransom payments in that year were $160m (£106m), a considerable financial blow for many companies.

But the maritime security industry is not without its critics. Kaye Stearman of the Campaign Against Arms Trade said there was widespread concern about the use of private security companies. A UN working group on mercenaries has warned that a lack of regulation for armed security on ships, together with an absence of robust reporting for incidents at sea, create human rights risks.

"Although hundreds of companies have signed up to an international code of conduct for private security providers, this is a voluntary agreement," said Stearman.

"In the case of Sri Lanka, there is always the possibility that weapons supposedly licensed for use in anti-piracy operations will be used within Sri Lanka. Once weapons are licensed and despatched abroad, the exporter has no control over how or where they are used."

Gibson argues that the British government's decision to let the industry self-regulate is working. "It was a completely unregulated sector but there are an awful lot of ex-British military involved. They brought with them that conscience of trying to do the right thing, operating within the rule of law and respecting human rights," he said. There is now a specialist City & Guilds qualification for maritime security operatives, and an international standard has been created, audited by a third party. From September there will also be an international code of conduct for security providers and a Geneva-based international association charged with oversight.

A spokesman for the Foreign Office said that the government operated one of the most rigorous arms export control regimes in the world.

He added: "This includes requiring end users to provide a declaration that states the intended use and location for the products. They are also assessed carefully against the risk that the goods might be diverted or re-exported to undesirable end uses or end users."

The Sportsman Gun Centre – which is run by businessman Gary Lamburn and has three outlets in the West Country, Dorset and Wales – was not forthcoming about who placed the orders for the 1,000 weapons it was granted licences to export to Sri Lanka. The company may have yet to act on the licences, the deals may have been cancelled, or Sportsman may simply not accept they are exporting to Sri Lanka when the arms are for use at sea by a private company.

"It is our policy to respect client confidentiality so we are unable to comment on specific transactions," the firm said. "However, we have never exported any weapons to Sri Lanka."