Ian Austin, a vocal Corbyn critic and former Labour MP who left the party citing its problems with antisemitism, questioned “whose side would we be on” had Corbyn got into Downing Street following last month’s election.
Mr Corbyn had branded the assassination of Qassem Soleimani, the head of Iran’s Quds Force, as a “dangerous escalation”, prompting Mr Austin to respond on Twitter to list his criticisms of Iran.
Mr Austin, who represented Dudley North before leaving Parliament, was so opposed to Mr Corbyn becoming Prime Minister he urged voters back the Tories in the general election.
Labour was contacted for comment on Mr Austin’s remarks.
Qassem Soleimani was killed in a US air strike in Iraq.
The US accused him of “actively developing plans to attack American diplomats and service members in Iraq and throughout the region”.
The US also said Soleimani approved attacks on the US Embassy in Baghdad earlier this week, and blamed Iran for attacks targeting tankers and the September strike on Saudi Arabia’s oil industry.
Earlier today, Mr Corbyn said: “The US assassination of Iranian general Qassem Soleimani is an extremely serious and dangerous escalation of conflict in the Middle East with global significance.
“The UK government should urge restraint on the part of both Iran and the US, and stand up to the belligerent actions and rhetoric coming from the United States.
“All countries in the region and beyond should seek to ratchet down the tensions to avoid deepening conflict, which can only bring further misery to the region, 17 years on from the disastrous invasion of Iraq.”
Mr Corbyn has previously been criticised for his past appearances on Iran’s Press TV.
“We have always recognised the aggressive threat posed by the Iranian Quds Force led by Qassem Soleimani,” he said.
“Following his death, we urge all parties to de-escalate. Further conflict is in none of our interests.”
Boris Johnson has been criticised by a Labour shadow cabinet member for not speaking about the strike.