The National Freedom Front's Wimal Weerawansa warned that the United National Party and Tamil National Alliance would join together and allow an international war crimes probe to take place in Sri Lanka.
Speaking at a press conference alongside former president Mahinda Rajapaksa, Mr Weerawansa said that over 40 military officers and two civilian leaders were at risk of facing war crimes charges.
"If the UNP forms a government with the help of the TNA it would have to grant the wish of the latter which has called for an international war crimes probe," he warned.
"This has been part of their strategy and the onus is on the people to ensure the safety of these leaders who fought for us to elect the UPFA."
Despite international calls for Sri Lankan leaders to face trial over massacres that took place in 2009, Sinhala political leaders, including the current president, have vowed to protect Sri Lankan officials from facing an international probe.
See our earlier posts:
Domestic probe will have foreign hand - UPFA (02 Aug 2015)