Singh, who left for Tehran this evening to attend the NAM Summit, is learnt to have issued the communication before his departure.
After the exit of Pranab Mukherjee from the Council of Ministers, Antony seemed to have got the number two position in the government.
An indication to this effect had come when Antony was allocated the seat next to Prime Minister Manmohan Singh for the first time at a July 12 cabinet meeting here.
Antony was allocated the seat to the right of the Prime Minister, where Mukherjee used to sit.
The seat next to the Prime Minister on the left side is occupied by Cabinet Secretary Ajit Seth as usual.
Earlier, Mukherjee, who resigned as Finance Minister on June 26 to contest the Presidential poll, was the de facto number two in the government. He would stay in Delhi whenever the Prime Minister had to go abroad on official visits.