Abramovich and Ukrainian MP may have been poisoned this month

Russian billionaire and Rustem Umerov had symptoms consistent with poisoning earlier in March, says source

• Analysis: a familiar story, but truth may never be known about latest Russian poisoning'

The latest round of negotiations between Russia and Ukraine, set to begin in Istanbul on Tuesday, have been overshadowed by claims that the Russian billionaire Roman Abramovich and other members of an informal Russian-Ukrainian negotiating group suffered symptoms consistent with poisoning during an informal round of talks earlier this month.

The allegations surfaced on the eve of the first face-to-face peace talks in weeks, amid fears that the Kremlin was not ready to compromise, despite the stiff resistance its forces have encountered since the invasion began.