By Adrian Croft and Justyna Pawlak
BRUSSELS (Reuters) - European Union envoys will make another attempt this week to break a deadlock over jailed Ukrainian opposition leader Yulia Tymoshenko that has jeopardized plans by the EU and Ukraine to sign a landmark trade and cooperation deal.
The envoys, former Polish President Aleksander Kwasniewski and Irish politician Pat Cox, will return to Kiev for a session of parliament on Tuesday that will try again to pass a law allowing the 52-year-old Tymoshenko to go to Germany for medical treatment of her chronic back pain.
The case of ex-prime minister Tymoshenko, a fierce opponent of Ukrainian President Viktor Yanukovich who was jailed in 2011 for seven years for abuse of office, has become symbolic for the EU of the "selective justice" that the bloc wants ended before it will sign the accord with Ukraine.
The EU envoys say Tuesday's session is the last chance for parliament to pass the law before a November 28-29 summit in the Lithuania at which the EU and Ukraine hope to sign the agreement that would mark an historic shift westwards for the former Soviet republic, away from Russia's orbit.
Yanukovich signaled on Thursday that he would let Tymoshenko go to Germany, but only if she went there as a convicted person, falling short of the pardon some EU governments would like to see.
This implied that she would have to return to Ukraine to complete her sentence after treatment and would be ruled out from taking part in political activity.
Cox said the EU envoys were ready to spend "as long as is necessary, with as many people as is necessary, to do whatever is necessary to secure success". He said they were ready to help and advice Ukrainian politicians on finding a solution.
Parliament failed last week to agree on a draft law that Yanukovich has to sign into law for it to take effect.
Without movement on Tymoshenko, the 28-nation EU is unlikely to sign the deal with Ukraine, depriving the Vilnius summit of its centerpiece and blowing a hole in the EU's broader strategy of building closer ties with ex-Soviet states.
Failure to sign in Vilnius would also be a serious blow for Yanukovich, who has set integration with Europe as his main foreign policy aim for Ukraine, a country of nearly 46 million people with a $330 billion economy.
But it would please Moscow which has used trade sanctions and the threat of disruption to energy supplies to dissuade Ukraine, Moldova, Georgia and other former Soviet republics from moving closer to the EU. Russia has urged these states instead to join a Russian-led customs union.
EU foreign ministers meet in Brussels on Monday and will discuss Ukraine's progress in implementing the EU's conditions for signing the agreement.
They had been due to decide on Monday whether Ukraine had met the conditions but the Ukraine parliamentary session on Tuesday means that decision is likely to be delayed, possibly even until the Vilnius summit itself.
One EU diplomat said he thought Ukraine would not sign, sparing the EU a tough decision on whether to accept the deal.
"In Vilnius, there will be no showdown because Ukraine will put us out of trouble, as they will not sign. They will choose for us," the diplomat said.
A last-minute EU decision on whether to sign in Vilnius, with the possibility of splits among EU governments being played out in the full glare of international publicity, is something that EU officials had been desperate to avoid.
EU officials say there is no "plan B" if no agreement can be reached in Vilnius and that once momentum towards a deal is lost, it may be hard to recover.
(Additional reporting by Francesco Guarascio in Brussels and Richard Balmforth in Kiev; Editing by Alison Williams)