The Israeli military said Wednesday that one of the bodies handed over by Hamas as part of an exchange for Palestinian prisoners was not a former hostage.
After overnight forensic tests on four bodies returned on Tuesday, the army said medical officials concluded that one "does not match any of the hostages".
"Hamas is required to make all necessary efforts to return the deceased hostages," the military warned.
Since Monday, under a ceasefire agreement brokered by US President Donald Trump, Hamas has handed back 20 surviving Israeli hostages and eight bodies -- one Nepalese, six Israelis and a now unidentified eighth.
Separately, a Gaza hospital said it had received the remains of 45 Palestinians handed back by Israel.
News that one set of remains was not of someone on Israel's 28-name list of deceased former hostages will increase domestic pressure on Prime Minister Benjamin Netanyahu's government to restrict aid access to Gaza.
According to public broadcaster KAN, Israel was ready to reopen the territory's Rafah border crossing to Egypt on Wednesday.
But hardliners like far-right National Security Minister Itamar Ben Gvir has called for aid supplies to Gaza to be cut if Hamas fails to return the remains of soldiers still held in the territory.
Two of the Israeli bodies returned so far have been confirmed to be soldiers.