"There may come a moment when the roof can no longer hold," Alexander Rumyantsev said in Moscow.
Chernobyl was the site of the world's worst nuclear accident when a reactor exploded April 26, 1986, spewing radiation across a vast swath of then-Soviet Ukraine, Russia, Belarus and Western Europe.
Rumyantsev said the shell over the damaged reactor was constructed hastily "under the most difficult" conditions and has gaps that threaten to leak radiation.
He also doubted that Ukrainian officials were carrying out the necessary scientific monitoring of the site.
"No one is conducting tests on the damaged walls," Rumyantsev said, adding that a stronger concrete shelter could be built over the existing sarcophagus.
International donors have pledged hundreds of millions of dollars to build a new shelter but construction is not expected to start before next year.
Rumyantsev said he is well acquainted with the deficiencies of the Chernobyl shell because he worked for years at Moscow's Kurchatov nuclear institute, which has monitored the plant since Soviet times.