Arun Singh, India's Ambassador to US, is due for retirement and it is understood that Sarna's name has been finalised to succeed him.
It is learnt the India's High Commissioner to Sri Lanka Yashvardhan Kumar Sinha may replace Sarna in the UK.
Sarna, an Indian Foreign Service (IFS) officer of the 1980 batch, was serving as Secretary (West) in the Ministry of External Affairs before he was posted to London in January.
The 59-year-old diplomat was among the longest-serving spokespersons of the MEA. He had held the post between 2002 and 2008.
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Sarna is an author of many fiction and non-fiction books, with the most recent being 'Second Thoughts: On Books, Authors and the Writerly Life' released last year. He was also India's Ambassador to Israel from 2008 to 2012.
For two years from August 2012, Sarna had served as additional secretary in-charge of international organisations in the MEA.
Sarna had served at various Indian missions, including in Moscow, Warsaw, Tehran, Geneva, Thimphu and Washington.
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