Sky News reported the information, citing Sunak’s remark made in Britain's parliament.
The PM said Russian military used "multiple missiles" in the attack late last month.
"Thanks to declassified intelligence, we know the Russian military targeted a civilian cargo ship in the Black Sea with multiple missiles on the 24th of August," Sunak said as he was updating MPs after his trip to the G20 summit in New Delhi last week.
The missiles had targeted a Liberian-flagged cargo ship in Odesa port and were successfully shot down, the Foreign, Commonwealth and Development Office (FCDO) said in a statement. The missiles included two Kalibrs fired from a Black Sea Fleet missile carrier. Ukrainian air defense thwarted the attack, with neither of the Kalibr missiles hitting their target.
Britain’s prime minister added in just one month, Russia has destroyed over 270,000 tonnes of grain - enough to feed a million people for a year.
He said that at the G20, he and fellow leaders "called out the human suffering caused by Putin's war". Sunak reiterated that Ukraine has the right to conduct trade through international waters and a moral right to feed the world with grain.