Home secretary Theresa May could make Britain look "weak," the opposition spokesman said.
The enquiry into the death of the former Russian KGB officer has been closed.
The verdict on the Litvinenko murder case could strain relations between London and Moscow.
Yuri Shvets testified yesterday as part of the inquiry into former Russian intelligent agent Alexander Litvinenko's death.
According to Pavel Khodorkovsky's Institute for Modern Russia the KGB practice of killing defectors is back.