Four men who were being held on charges of spying and terrorism have been released after five months in captivity

Four British men held on spying and terrorism charges in the Horn of Africa state of Eritrea have been freed after five months in captivity.

The two ex-Marine guards and two civilian crew members, working for an anti-piracy security firm, were expected to land at Heathrow airport late tonight.

But questions remained over the security contractors' activities in the isolationist country, including official allegations that they set up a "military base" on a remote island and stashed sniper rifles, silencers, pistols and "poison-tipped" bullets "intended for perpetrating acts of terrorism and sabotage".

The men worked for British maritime security company Protection Vessels International, which said they made an unscheduled stop in Eritrea last December owing to rough weather while en route to provide security for ships in an area where piracy is rife. PVI claimed they were arrested because of "confusion over fuel payments".

But Eritrea's government insisted that the men were detained as they tried to leave Eritrean waters without permission, and said its searches then uncovered a weapons cache on the island of Romia. The government alleged that a total of 21 PVI employees were involved in crimes "against Eritrean sovereignty", and that four were caught while trying to escape.

"On 19 December, 21 members of the PVI (including those four detained) infiltrated and deployed different types of weapons, poison-tipped bullets, bulletproof vests, specialised communication equipment and infrared night-vision binoculars in the sovereign Eritrean island of Romia," a government press release said last week.

"There is high possibility that such military hardware is intended for perpetrating acts of terrorism and sabotage. Hence, the members are accountable for infiltrating into the sovereign Eritrean island of Romia and stashing weaponry, for orchestrating acts of espionage and terrorism."

The statement also claimed that 11 seamen stayed in hiding on Romia for four days, along with military hardware, equipment and communication facilities. "These seamen were given directives to keep guard in twos and to pose as 'tourists keen to observe sharks'," it said, proving that the PVI had been using Romia "as its military base and arms depot".

PVI employs former British marines to provide armed on-board security and escort vessels for shipping operating in the Gulf of Aden and the Indian Ocean. It has denied Eritrea's claims.

Paul Gibbins, a company spokesman, said this was the worst event in the company's two-year history and a full investigation would be conducted into what had gone wrong.

"We need to find out what the pinch point was. Was there a confusion over the fuel? Was there an issue over the vessel's departure? We just don't know at this stage," he said. "The whole episode has been a series of unfortunate events."

Gibbins denied that the company was involved "any espionage or acts of assassination". He said that the ship had been forced to stop in Massawa for a crucial fuel resupply and equipment repair.

But, he insisted, the company had worked hard before it docked to ensure that the ship's presence was not regarded "as a threat or hostile action".

"Christopher Collison was dispatched to Massawa to liaise with the local shipping agent and the port authority," he said. "He was there for four days, explaining and negotiating. We had only intermittent contact with him during that time, because mobile phones don't work in that area. We worked very hard to ascertain if it was safe to enter the port."

Gibbins said that, despite the lack of contact, the company was eventually forced to make the decision to give the ship permission to enter the port. However, he said, it had tried to "desensitise" and "detune" the presence of both the ship and its crew by unloading the weapons they carried as part of their regular duties on Romia, an outlying and uninhabitated island, before landing.

"We did everything we could to make it clear our presence was not misconstrued as a hostile act," he said.

The ship spent five days at Massawa, but was eventually forced to leave before it had fully refuelled. "We needed 15 tonnes of fuel but could only get 1,500 litres before we were forced to leave to meet an engagement to provide security for clients," said Gibbins. "In hindsight, it's true we could have done things better but, given the time constraints and the immediacy of the decisions that needed to be made, we worked hard to facilitate safe entry to the port. We do regret those unfortunate events and apologise to everyone concerned."