Ukrainian House Speaker Volodymyr Rybak last week called on a high court to annul the parliamentary mandate of Serhiy Vlasenko.
A spokesman for EU foreign policy chief Catherine Ashton and EU Enlargement Commissioner Stefan Fule said they were following the matter closely.
"They call on the Ukrainian authorities to address this situation so as to avoid creating any perception of misuse of the judiciary for political purposes," a statement read.
Rybak said Vlasenko can't serve both as a lawyer and a lawmaker, reports Bloomberg News. Vlasenko said he worked in defense for former Prime Minister Yulia Tymoshenko but stopped practicing law in 2008.
Tymoshenko was sentenced to seven years in prison in 2011 after being convicted on charges she abused her authority when she helped broker a deal with Russian energy company Gazprom.
The European Union said the Tymoshenko case, judicial imbalance and the conduct of October parliamentary elections were in the way of signing formal association deals with Ukraine.
"Legal proceedings which come months after the confirmation of the final election results raise political and legal concerns," the statement from the EU leaders read.