Seven UK aid workers, who were planning to take aid to Gaza, are among a group believed to be being held captive on a ship in the Libyan sea.
The Strofades IV was taken out of Derna Harbour after volunteers on the Road to Hope convey had a dispute with the Greek captain over a payment.
Tauqir Sharif from Walthamstow and Kieran Turner from Liverpool are on-board.
The Foreign Office is urging Libyan authorities to resolve the situation.
The ship is now in international waters and is surrounded by Libyan navy ships and two fighter jets, the PA news agency reported.
Ellie Merton, London liaison for the convoy, told the BBC: "We're extremely worried for the guys on the boat."
She said there were also two Irish people, an Algerian and three Libyans - two policemen and a port manager who were not part of the convoy - on the boat.
She said the captain became irritable in the port area over payments but the group had given the money in cash to a shipping agent.
"We're trying to provide the money that he wanted which was $75,000 (£46,497.32). We were trying to provide it in cash if he needed it because he said he didn't trust electronic transfers."
She said the rear of the ship was partly closed and a convoy vehicle was stuck on it and this was of great concern to the volunteers.
She said she did not believe the ship was seaworthy and that the captain had been refusing to turn around.
Those who are on the ship are aged between 24 and 60 years old.
Mr Sharif was also on the Gaza-bound aid ship Mavi Marmara which tried to breach an Israeli naval blockade in May this year.
Ms Merton had said that she was astonished by the situation.
She added: "As far as I know, the boat owner became really aggressive over a payment for something, tried to shut the back end of the boat and moved away from the pier really fast.
"The boat was still attached to the quayside and as he drove he yanked the ropes off, there were bits of concrete flying everywhere.
"We have lost telephone contact with those who are still on the ship against their will - without any paperwork, passports or authority to leave Libya, possessions or, when we last heard, food or water."
A spokesman for the Foreign Office said: "We are aware of the incident at Derna Port in Libya, and have been in close contact with the convoy organisers.
"Our Embassy in Tripoli has been urging the Libyan authorities to resolve the situation rapidly and ensure that those caught on the ship are returned to safety."