''I voluntarily resigned and I have been relieved,'' Lambah, who took charge of his assignment last November, told PTI in Delhi.
He played an active role in the reopening of the Indian embassy in Kabul in December last soon after the interim regime under Hamid Karzai was installed.
Lambah denied that he had put in his papers following a change of guard at the ministerial level with Yashwant Sinha swapping places with Jaswant Singh.
Lambah's appointment, made at the instance of Jaswant Singh, had the Union Cabinet's approval and was open-ended.
''I could have continued for a year or more. But the job for which I was assigned has been completed,'' he said.
''In my judgement, my work is over,'' Lambah said adding that a transitional government was in place in Afghanistan and that the Indian embassy had become operational.
Lambah, former Indian ambassador to Pakistan, had represented the country during international negotiations for the establishment of an interim administration to replace the ousted Taliban regime.
Lambah had also visited Tokyo, Geneva, Rome and the US to coordinate India's approach as part of international efforts to rebuild Afghanistan.
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