Vinayagamoorthi Muralitharan, also known as Colonel Karuna, was sentenced at Isleworth Crown Court in London after admitting the charge at a lower court on December 24 last month.
He was charged under the Identity Cards Act 2006 after entering Britain illegally, a Home Office spokeswoman told AFP.
Karuna was reportedly arrested in London in November.
Human rights group Amnesty International has called for British authorities to investigate whether he can be prosecuted here for international war crimes, including abducting and using child soldiers.
Karuna, who has consistently denied any wrongdoing, was the de facto number two of the ruthless Tamil Tigers -- renowned for their use of suicide bombers and child fighters -- before he defected in 2004.
After that he reportedly worked with Sri Lanka's armed forces to drive out the Tamil Tigers from a large rebel enclave near the eastern lagoon town of Batticaloa, one of the biggest government successes of 2007.
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The United Nations