"If Tymoshenko is not released by the start of the Euro in June, the German footballers will then most likely have to play without Merkel being in attendance," the magazine said, without naming any sources.
Last October, the European Union saw Tymoshenko's jailing as politically-motivated and has criticised her conviction (see background). Ukraine is co-hosting the June 8-July 1 tournament together with neighbouring Poland.
Merkel usually travels to attend important matches involving the German national team, Reuters reported. Der Spiegel said if Merkel refused to attend, other cabinet members were likely to follow suit. A spokesman for the German government declined to comment on the report.
German President Joachim Gauck said last week he had cancelled a planned visit to Ukraine next month. The former human rights activist had been due to attend a meeting of central European heads of state in Ukraine's Black Sea resort of Yalta.
On Friday, Merkel's spokesman Steffen Seibert said a decision on whether the chancellor would attend matches in Ukraine depended on the situation.
"Obviously further developments in Ukraine and the Tymoshenko case would play a role in the decision," he said.
Other EU nations have also been alarmed. On Sunday, Italy's Foreign Minister Giulio Terzi said he was following the case with "growing concern".
"The news...that Ms Tymoshenko was said to be recently faced with physical violence during a transfer from prison to hospital can't help but raise serious alarm," he said in a statement.
Berlin has repeatedly offered to treat Tymoshenko, who suffers from back pain, in a German hospital.
A poll by German newspaper Bild am Sonntag found that 52% of Germans are in favour of Merkel and her ministers staying away from matches in Ukraine.
The poll also found 50% of Germans were opposed to Euro matches taking place in Ukraine in the first place.
Terrorism threat?
A series of bomb explosions in Ukraine added to the tensions. Four bombs left in trash cans in various locations in Dnipropetrovsk, an industrial center of over one million inhabitants in eastern Ukraine, exploded in quick succession around midday on Friday, injuring 30 people, including 11 children.
Ukrainian President Viktor Yanukovich promised a swift probe into the blasts, which the country’s prosecutors said were an "act of terrorism".
Polish president Bronisław Komorowski voiced worries for the football championship his country is co-hosting with Ukraine.
"Of course this causes concern, including in Poland because of the approaching Euro 2012 tournament, [about] a worsening atmosphere… in a country with which we are organising this great sporting event," Komorowski said, as reported by the Voice of Warsaw.
Poland’s Internal Security Agency said that for now it was not recommending that the authorities in Warsaw should warn of an increased level of terrorist threat. It added in a statement that “there is no reason to link the attacks on Ukraine with the current level of terrorist threat to Poland.”
Media in Poland have speculated that the explosions may have been the work of feuding criminal gangs.
Ukrainian soccer federation chief Hryhory Surkis said that those who planted the bombs were "accomplices to an attack on the image of our country ahead of Euro 2012." Euro 2012 matches will not be played in Dnipropetrovsk itself.
UEFA, European soccer's governing body, said it remained confident that security measures taken by Ukraine would ensure a "smooth and festive" tournament, despite the bomb blasts.
