Image caption The firm the men work for has said the incident was a misunderstanding

Four Britons held in Eritrea on spying and terrorism charges since December have been released, the Foreign Office (FCO) has said.

The four men - who all worked for the same security firm - are believed to be on their way back to the UK.

The FCO said it was very grateful to the government of Qatar "for helping facilitate their return".

But it said it remained concerned that the Eritreans had not responded to requests for consular access.

The four men had been on board a merchant vessel that stopped at a port in the east African country to pick up supplies when they were detained in December 2010.

The Eritrean government said the men were detained as they tried to leave Eritrean waters without permission and searches then uncovered a weapons cache.

But private security firm Protection Vessels International (PVI) said they were en route to provide security for ships in the region.

Sanctions

A Foreign Office spokeswoman said: "We are pleased that the four British nationals have now been able to leave Eritrea and can be reunited with family and friends. We are very grateful to the Government of the State of Qatar for helping facilitate their return.

"We remain concerned however that at no time did the Eritreans respond to our requests for consular access and will continue to raise this matter with them."

As part of efforts to secure the men's release, Foreign Secretary William Hague had ordered UK ambassadors around the world to raise concerns with their Eritrean counterparts.

In response to the lack of consular access to the four Britons, the Foreign Office had also imposed sanctions on Eritrean diplomats in the UK.

On 4 May the Eritrean ambassador was informed that, with immediate effect, Eritrean diplomats and visiting officials would be required to obtain the FCO's written permission before travelling beyond London.

And on 20 May the ambassador was informed that aspects of the collection of a tax levied by the Eritrean government on Eritreans living in the UK may be unlawful and in breach of the Vienna Convention on Diplomatic Relations.

The ambassador was told that, until it was demonstrated otherwise, the embassy should immediately suspend all activities relating to the collection of the tax.