a legal battle on Tuesday to stay in Great Britain despite the fact that Croatia sought his extradition, Reuters reported.
Judge Timothy Workman told a court in London it was so long since the alleged offences took place there were doubts about whether Spanovic would receive a fair retrial if extradited.
"I find that it would now be both unjust and oppressive to extradite the defendant to Croatia," Workman said.
Spanovic had "a reasonable expectation that he could live freely in this country and ... he has done so in employment, supporting his family and without committing offences," he said.
The 44-year-old Spanovic was accused of torturing and killing civilians, pillage and arson in 1991, at the outset of the Serb rebellion against Croatia's independence, the agency said.
He was tried in absentia by a court in the central town of Sisak and sentenced to 20 years in jail for war crimes.
The Croatian Justice Ministry, which had issued an international arrest warrant for Spanovic, has promised to allow him a retrial if he demands it.
Workman told the court that the alleged offences occurred during a period of civil war "in which inevitably evidence will be hard to find or reconstruct".
Spanovic entered Britain in 1998 and was granted leave in 2000 to stay in the country. The prosecution told the court that Croatia would appeal against the ruling, Reuters said.