The mother of one of the UK nationals detained in Russia following a Greenpeace protest has expressed her concern that some of the prisoners will be "lost" during transportation.
Sue Turner, mother of second engineer Iain Rogers, also described the consular support from the British authorities as "patchy".
The group of 28 activists and two journalists were being held in Murmansk but have embarked on a 27-hour train journey to St Petersburg around 800 miles away.
They are due to arrive at Russia's second largest city at noon local time tomorrow (8am GMT).
Ms Turner was concerned that some of the group would be lost during the journey after jailed Pussy Riot member Nadezhda Tolokonnikova was reported missing last week.
She said: "I'm just worried that they are going to lose some of them on the way to St Petersburg. I won't be really happy until they all arrive there and we know where all 30 are.
"They have lost a Pussy Riot member, she's disappeared. What's to say that they cannot lose some of our Arctic 30 while they are transporting them from Murmansk to St Petersburg and then onto maybe different prisons in St Petersburg.
"They are not necessarily going to be sent to the same place. We were told that they were hoping they would be sent to the same place but we have got no guarantee of that at all and no guarantee that their conditions are going to be any better."
When asked what motive the Russian authorities could have for losing some of the activists, she replied: "It would make life easier for them. They would have less people to worry about, less people to prosecute."
Ms Turner said: "The consular support initially was very patchy. I don't think it was good enough.
"It has improved since our last meeting in London and probably it will improve now the minister knows our feelings about the lack of communication, lack of honesty with the consular staff."
The relatives of father-of-three activist Phil Ball also met with David Cameron as they are from his constituency of Witney in Oxfordshire.
Mr Ball's partner, Jenna Saunders, said: "I felt it was quite positive. We very much met him as our MP and not as the Prime Minister.
"I felt he has taken an interest in Phil and is following the whole process quite closely."
Ms Saunders added that Mr Cameron described a telephone call to discuss the issue with Russian president Vladimir Putin as "quite positive".
But Mr Ball's brother, Steve, admitted that the influence of the Prime Minister was limited.
"He can have no grounds for optimism," he said. "The gentle influence that the British Foreign Office can have is being placed at the moment.
"Until we get a sentence there is nothing for us to react to."
The Britons being held are Mr Ball, 42, Mr Rogers, 37, from Exeter, video journalist Kieron Bryan, 29, from Devon, and activists Alexandra Harris, 27, originally also from Devon, Frank Hewetson, 48, from London, and Anthony Perrett, 32, from Newport.