Ukraine's former prime minister Yulia Tymoshenko, sentenced to seven years in prison for abuse of office, has been charged with tax evasion, theft and concealing foreign currency revenues, the State Tax Administration said on Friday.

The new charges indicate that she is less likely to go free in the near future despite calls from the West for her release, keeping Ukraine's ties with the European Union strained.

The charges are linked to her activities as head of gas trading firm United Energy Systems of Ukraine, a post she held in the 1990s, an administration statement said.

Her lawyer Serhiy Vlasenko dismissed them as baseless.

Tymoshenko was found guilty on October 11 of abusing her powers as prime minister in 2009 when she forced through a gas supply deal between Ukrainian state energy firm Naftogaz and Russia's Gazprom.

Tymoshenko has denied all charges against her and said her trial reflected a political vendetta waged by President Viktor Yanukovich, who narrowly beat her in the former Soviet republic's 2010 presidential election.

Echoing her position, the European Union has warned Ukraine it may not sign bilateral deals on political association and free trade if Tymoshenko remains in jail.

There is a possibility her conviction will be overturned by an appeals court or reclassified as a non-felony by parliament.

However, state prosecutors have said they could launch more cases against Tymoshenko and were studying her possible involvement in a 1996 contract killing of a parliament deputy -- which her supporters have denied.

Supporters of Tymoshenko, 50, say her health is deteriorating as she has been in a detention centre since early August and now has trouble walking.

A member of a state-appointed medical team tasked with assessing Tymoshenko's health said on Friday she had been told to stay in bed but a diagnosis of her illness had not yet been established, according to Interfax news agency.

Tymoshenko was a leader of the 2004 Orange Revolution which doomed Yanukovich's first bid for the presidency. She served twice as prime minister before losing the 2010 election to Yanukovich in a bitterly fought runoff.

Her supporters say he is seeking to extinguish her as a political force before a parliamentary election next October.

(Reporting by Olzhas Auyezov; Editing by Janet Lawrence)