Arun Kumar Singh, India's ambassador to US, will retire from the Indian Foreign Service in September. Sarna's name has reportedly been finalised to succeed him.
India's High Commissioner to Sri Lanka Yashvardhan Kumar Sinha may replace Sarna in the UK.
Sarna, an IFS diplomat of the 1980 batch, served as Secretary, West, at the ministry of external affairs before he was posted to London in January.
Sarna, 59, is also among the MEA's longest serving spokespersons, having held the post between 2002 and 2008.
He is likely to take up his new assignment in the midst of America's presidential election campaign.
Sarna has written several fiction and non-fiction books, the most recent being Second Thoughts: On Books, Authors and the Writerly Lifereleased last year. He was also India's ambassador to Israel from 2008 to 2012.
For two years from August 2012, Sarna served as additional secretary in charge of international organisations at the MEA.
Sarna has served in Moscow, Warsaw, Tehran, Geneva, Thimphu and Washington, DC.
In his 35-year diplomatic career, Sinha has served in India's diplomatic missions in South Asia, the Middle East, Europe and South America.