"The sudden and summary removal of the seniormost woman foreign service official by Modi government raises serious questions on its intent and administrative mechanism it is following", party spokesman Randeep Surjewala said.
This, he said, was especially because the development has come on the heels of similar summary terminations of SPG chief, DRDO Chief and IIM director.
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"It puts a serious question mark on the administrative consistency and efficacy of actions of the Prime Minister", he said, adding that this was more so because this "without reason removal" has happened after "supposedly successful" visit of the U S President.
Prime Minister and Foreign Minister Sushma Swaraj owe an explanation to the nation, he added.
"Is sacking of Foreign Secretary late retribution for her stand on Devyani Khobragade affair? Removal after a Presidential visit 'coincidental'? former Information and Broadcasting Minister Manish Tewari wrote on Twitter.
Government last night decided to replace Singh curtailing her tenure due to end in August and appointed India's Ambassador to the US S Jaishankar in her place.