A military analyst believes the missile blast in Poland was likely caused by one of Ukraine's old Soviet rockets malfunctioning.
US officials believe Ukraine accidentally fired the S-300 air-defence missile into Poland yesterday, killing two people.
Defence expert Jarosław Wolski thinks the Ukrainian rocket malfunctioned because it was four decades old.
He said Kyiv is relying on missiles from the 1980s to shoot down Russian rockets as the country has depleted its air-defence ammunition stocks.
Mr Wolski, 37, who previously taught national security at Poland's University of Wrocław, told The Mirror: "Ukraine has almost run out of S-300 air-defence system rockets.
"Their stocks are very low and they are now using really old rockets which malfunction.
"A typical rocket has a shelf life of between 15 and 25 years. Unfortunately Ukraine doesn't have the money to buy new rockets so they use old ex Soviet Union stuff.
"The rocket that landed in Poland was probably made in the 1980s."
It comes after Poland's President Andrzej Duda said the rocket that hit the village of Przewodów was a S-300.
Mr Duda added: "From the information that we and our allies have, it was an S-300 rocket made in the Soviet Union, an old rocket and there is no evidence that it was launched by the Russian side.
"It is highly probable that it was fired by Ukrainian anti-aircraft defence."
Stanisław Iwanejko, who narrowly avoided being killed in the blast, said: "I was in the barn when I heard two strange whistles, as if something flew over my barn. It was very loud, a thousand decibels."
He identified the people who were killed as two men aged 60 and 62.
Stanisław Iwanejko said his friends were bringing the corn in from the field when they died.
It was initially feared the rocket may have been launched by Russia, sparking fears of a potential world war as Poland is a member of NATO.
Although it appears Ukraine fired the rocket into Poland, Rishi Sunak still blames Russia for the incident.
Mr Sunak said it took place against a barrage of more than 80 Russian missiles launched against civilian infrastructure in Ukraine on Tuesday.
He added: "Over the course of [Tuesday], Russia rained down something like over 80 different missile strikes on the Ukrainian people on their civilian infrastructure.
“That is a fact. It's barbaric, and it's wrong. And we shouldn't forget that.
"I think it shows utter contempt for the international rules-based system."
Mr Sunak added: "We should all be clear - none of this would be happening if it weren't for Russia's invasion of Ukraine."
In a G20 session, the PM told world leaders that the series of missile attacks by Russia “deserves to be condemned by us all”.
