The 2012 European Championships were heading towards a diplomatic fiasco on Monday after more EU leaders said they would join Germany in a boycott of the event next month unless Ukraine freed the opposition leader, Yulia Tymoshenko.

On Sunday, Angela Merkel said that she and her cabinet would not attend any games played in Ukraine, which is co-hosting the tournament with Poland, unless the human rights situation under President Viktor Yanukovych improved.

On Monday, the president of the European commission, José Manuel Barroso, and Viviane Reding, the commissioner for justice, said they would not be travelling to Ukraine either. The Czech president, Vaclav Klaus, announced he was cancelling a visit to Yalta, while Germany's president, Joachim Gauck, called off a trip to the same central European leaders' summit last week.

An European commission spokeswoman said that "as things stand" Barroso had "no intention of going" to Euro 2012. She described Tymoshenko's predicament as "a very, very serious situation". "It gives rise to very serious concern," she added.

Tymoshenko, the former prime minister, was jailed for seven years in October after what her supporters say was a politically motivated show trial. She has been on hunger strike since 20 April. On Friday, photos appeared showing bruises on her body. Tymoshenko claims prison guards assaulted her and punched her in the stomach. Ukrainian prosecutors say her injuries were self-inflicted.

Tymoshenko is currently being held in Kharkiv, in eastern Ukraine. The city is the venue for Germany's first group stage match against the Netherlands on 13 June. On Monday, after seeing the photographs of Tymoshenko's bruises, Sweden's foreign ministry summoned the Ukrainian ambassador and demanded an explanation.

The political debacle is a huge embarrassment for Uefa. Football's governing body in Europe had hoped that expanding the tournament eastwards would showcase the progress made by independent Ukraine since the collapse of communism. Instead it is now possible that Yanukovych could sit in the VIP box on his own, with European leaders shunning him.

All three of England's group-stage matches take place in Ukraine, with two in the eastern city of Donetsk and one in Kiev. England is in the same qualifying group as Ukraine, and plays the host nation in Donetsk on 19 June. The final of the tournament, which runs from 8 June to 1 July, also takes place in Kiev, Ukraine's capital. The Foreign Office said it was "keeping the situation under review".

Denis MacShane, the former Europe minister, said David Cameron should make it clear that there will be no official British presence at Euro 2012 unless Tymoshenko is released and given medical treatment for her severe back pain. If Kiev refuses to heed EU concerns, England's three matches should be played in neighbouring Poland, he suggested. Charles Tannock, a Tory MEP for London, said he supported MacShane's proposal.

The criticism from western Europe has provoked a sharp response from Kiev. On Sunday, foreign ministry press spokesman Oleg Voloshin accused Berlin of cold-war thinking. Other officials suggested that the Germans should refrain from meddling in Ukraine's internal affairs.

"I wouldn't like to think that the statesmen of Germany are capable of reanimating the methods of the cold war," Voloshin said, adding he hoped the threat of a boycott was a "newspaper canard".

Voloshin told the Guardian in an email that Ukraine was being unfairly punished. "Our position is very simple. Euro 2012 is about football not politics. It's impossible to solve any political issues through boycotting sporting events."

In an apparent swipe at Germany's cosy relationship with Russia, Voloshin said that "other countries in the region" allegedly had more problems with democracy than Ukraine, but that German politicians "kept mum" whenever they staged large sporting events.

MacShane said Kiev only had itself to blame for the unfolding mess. He described Tymoshenko's trial as "purely political vindictiveness to ensure there is no coherent opposition to this increasingly authoritarian regime." He pointed out that when in power, Tymoshenko had refrained from prosecuting Yanukovych, after his attempts to cheat during Ukraine's 2004 presidential election backfired and sparked the country's Orange Revolution.

Yanukovych won power in 2010. "It is Yanukovych who is putting the clock back by jailing his political opponents, rather than arguing with them or outvoting them in the ballot box," the Labour MP said.

Andrew Wilson, senior fellow at the European Council on Foreign Relations, said the Ukrainian government has little understanding of how the western media works. It had apparently not foreseen that by jailing Tymoshenko late last year, the country would face a major PR disaster during the summer of Euro 2012, he said. He added that Ukraine now risked being branded "Shrek" to Poland's "Princess".

Wilson said that Yanukovych, who was elected in 2010 and faces parliamentary polls later this year, was part of a clan from Donbas, a Russian-speaking industrial part of eastern Ukraine. This clan – now in government – was more comfortable using traditional Soviet methods, he said. "These guys from Dombas have this Old Testament political culture of smiting your enemies massively and showing who is boss. But if you do this in Kiev there are foreign policy consequences."

The EU has frozen conclusion of an association agreement between Brussels and Kiev because of Yanukovych's policies. Austria is also boycotting the summit of regional leaders in the Crimean resort of Yalta, with Baltic states expected to follow suit.

Germany had been negotiating quietly for weeks with the Yanukovich government, seeking to get Tymoshenko to a Berlin clinic for medical treatment. German doctors who have been allowed to treat her say she is suffering from a herniated spinal disc.

Over the weekend, a handful of German politicians floated the idea that Ukraine should now forfeit its host role, with its matches staged in Austria or Germany instead. With only 38 days to go until the tournament kicks off this is unlikely to happen. But Uefa officials do have an emergency plan B to transfer Ukraine's games to Poland. Work on plan B has intensified following four mysterious explosions last week in the Ukrainian city of Dnipropetrovsk, Tymoshenko's home town. Some 27 people were injured.

Last week, meanwhile, Rebecca Harms and Dany Cohn-Bendit, the Greens leaders in the European Parliament, wrote to Michel Platini, the Uefa head and former French national captain, demanding that the association take a stand on the Ukrainian political situation. "There has to be a clear public statement now from Uefa and the national football associations on the intolerable conditions in Ukraine before the tournament opens," they wrote. "It's inconceivable that we follow the tournament in the stadiums in Kharkiv, Kiev or Lvov while Yulia Tymoshenko sits in prison nearby."

Platini said at the weekend that Ukraine could not be barred from hosting the championship just because it was "less stable" than west European democracies. "It must be clear that there is no tolerance for human rights abuses and arbitrary justice in the football associations and among the players," the Greens leaders wrote.

Pressure is mounting in other countries too. On Sunday, Italy's foreign minister, Giulio Terzi, expressed "increasing worry" over Tymoshenko's situation and asked Ukrainian authorities to shed "full light" on the case. In a letter published on Sunday in the Rome daily Messaggero, Silvio Berlusconi, the former prime minister, called the campaign to free Tymoshenko "a political battle".

Berlusconi wrote: "I remain convinced that Kiev authorities have a lot to gain if Ukraine presents itself as host of a large sports event with a decisive step forward in the field of human rights."