Ben Emmerson QC - Marina Litvinko's lawyer has severely criticised the government's decision. He will seek judicial review.
He calls it "a repeated catalogue of broken promises[...] disastrous catalogue of indecision at the heart of government. [The government] has been paralysed by indecision".
Mr Emmerson says his widow and her son have been treated with "utter contempt" and in "the ultimate shabby way". He called it a "catalogue of disrespect".
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Rex Tillerson is president of Exxon Mobil and has close business links to Russian President Vladimir Putin.