65-year-old Fonseka, whose Democratic Party signed an agreement to join the ruling United National Front for Good Governance, said he has "nothing to hide" and was willing to face a war crimes inquiry.
"I have always said that I am ready to face any inquiry," Fonseka told reporters here. "We have nothing to hide. I feel that the allegations must be investigated."
The country's first five-star Field Marshal, Fonseka, is expected to become a member of parliament next week and a minister, sources said.
His comments about a war crimes inquiry come ahead of a visit to the island by the UN Human Rights High Commissioner (UNHRC) Zeid Raad Al Hussein this week.
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The UNHRC has asked for an investigation into human rights abuses by Fonseka's troops and the LTTE during the final phase of the nearly three decade-long war that ended in 2009.
Sri Lanka has opted for a domestic inquiry while Hussein has asked for an international investigation.
The former general had a fallout with Rajapaksa, under whom he had served, following which he was charged in a number of cases, ranging from corruption to engaging in politics in uniform.
He was also sentenced to 30 months in jail after having mounted a failed bid to unseat Rajapaksa in his January 2010 re-election.
Fonseka, who was nearly assassinated by a LTTE suicide bomber in 2006, was pardoned by Maithripala Sirisena days after he assumed office as the new president last year. All ranks and medals denied to Fonseka by the Rajapaksa regime along with his pension were restored.