As Ukrainian film director Oleg Sentsov enters his second month of a hunger strike in a Siberian prison, prominent Soviet dissident Sergei Kovalev has told The Independent that he sees direct parallels with the fate of his friend, the late human rights campaigner Andrei Sakharov.

Sakharov, a Nobel-prize-winning physicist, spent several months on hunger strike between 1984 and 1985. For most of that time he was force fed: first intravenously, then through a tube clamped onto his nose. Mr Sentsov, controversially sentenced to 20 years on a prison colony over disputed terrorism charges, faces a similar prospect.

He will not be allowed to die during the World Cup, his lawyer, Dmitry Dinze, has told The Independent.

Force-feeding seems imminent, and the protest looks to overshadow the start of the tournament in two days’ time.

On Sunday, with his health deteriorating, and with the opening of the tournament in sight, Mr Sentsov was moved to a medical ward. He is now under the constant watch of doctors, having lost more than 10kg, Mr Dinze said.

At least three other prisoners across Russia have joined Mr Sentsov in his hunger strike. One, Mr Sentsov’s co-defendant Olexandr Kolchenko, gave up a little over a week in. He had lost 8kg from an already slender figure and began fainting. Ukrainian Alexander Shumkov remains 19 days into a hunger strike, and Stanislav Zimovets, imprisoned in the anti-Putin protests of 2012, is 16 days in.

Mr Sentsov shows no sign of giving up on his campaign, which, he says, is not to secure his own freedom but that of 64 others.

In a confident letter to an activist friend, dated 7 June, received at the weekend, he said prison staff were already under no illusions.

“Everyone understands that I won’t step back,” he wrote.

The addressee, Tatiana Schur, a member of the Prison Oversight Commission in Chelyabinsk, told The Independent that certain aspects of the letter worried her. The text had obviously passed prison censors, so it was important to read between the lines, she said.

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“There are two things that are unusual,” she explains. “First, he has never urged us to write about his case before. This is out of character. Second, he mentions that doctors are worried about his health. This suggests a forced intervention is coming.”

On Saturday, Ukrainian president Petro Poroshenko and Russian leader Vladimir Putin held a rare telephone conversation that touched upon the Sentsov case.

The two agreed to allow human rights representatives to visit respective prisoners in Ukraine and Russia. This itself was considered a breakthrough since Russia has refused to even recognise Mr Sentsov as a Ukrainian citizen.

A native of annexed Crimea, the director has refused to take a Russian passport.

But the prospect of major concessions from the Kremlin remains distant. In his televised “direct line” with voters on 7 June, Vladimir Putin explicitly ruled out an exchange with a Russian journalist being held by Kiev on treason charges. “These are different things and not comparable,” he said.

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In 2015, a Russian military court found Mr Sentsov guilty of two acts of arson. The crimes were supposedly committed during the Crimean annexation but many have interpreted the harsh 20-year sentence as direct punishment for the director’s views and activism.

The testimony of people on the ground, at least, is not entirely consistent with the Russian version of events.

The Independent spoke to several people who knew Mr Sentsov in 2014, around the time he was alleged to have been an extremist. He was involved in the evacuation of stranded Ukrainian military and their families, they all said. He was forthright in his pro-Ukrainianism. But if anything, the director was someone who tried to push activists away from extreme acts, they said.

“We felt safe around him,” says Olexandra Dvoretska, an activist now living in Kiev.

“He warned people away from acts of aggression, saying that this is what Russia wanted – to show Ukrainians as radical fascists.”

It is unclear if Mr Putin truly believes the Ukrainian director to be guilty of anything.

We are back in those dark Soviet times again, and the West needs to think hard. My country is once again a threat to the world and to its own people. So far, the world has decided to close its eyes to everything apart from football, and that is shameful Sergei Kovalev, Soviet-era dissident

But the official position has helped ease the case away from domestic headlines. According to Dennis Volkov of the independent Levada Centre pollsters, the majority of the population do not even know who the director is, and those that are familiar with him think he is a terrorist who should do time.

Mr Kovalev, who spent 10 years either imprisoned in labour camps or in internal exile – sent to a provincial Russian town with no freedom of movement – told The Independent that it was too early to say if Mr Sentsov’s hunger strike would repeat Sakharov’s ultimate success.

The Soviet dissident eventually got what he asked for: that his wife, Yelena Bonner, be allowed to travel to the West for medical treatment. Within a year, he was also allowed to return to Moscow, where he would regularly meet with Mr Kovalev, himself just released from internal exile.

But Mr Kovalev says he rarely thinks about the hunger strikes when remembering his friend. What he remembers, he says, was the way Sakharov made regular appeals to the West – to contain Soviet military aggression and to support Soviet rights.

With all the world now focused on Russia, the time has come to repeat those appeals, he says.