Singh, in a letter to Home Minister P Chidambaram here, who he met, said he had made the recommendation for Khan at the behest of a Bihar Congress leader.
The Enforcement Directorate will also quiz Vijay Shankar Pandey, who was removed as UP Principal Secretary (Information), following questioning of the Bihar politician Amalendu Pandey in Mumbai.
The two will be questioned in connection with the passport procured by Khan and some investments done by him abroad.
In his letter, Singh said that on April 4, 1997 Amalendu Pandey approached him for issuance of passport from Patna for someone on compassionate grounds.
The reason given was the brother of the person seeking passport was seriously ill abroad and the presence of the applicant for looking after him was extremely essential.
"In view of the plea, explained by Amalendu Pandey, a known Congress leader, I referred the case to the then Minister of External Affairs I K Gujral on April 4, 1997...
"It is pertinent to mention here that I did not know the said applicant H Khan at all neither did I ever meet him. I only issued the letter for expeditious issue of his passport, believing the assertions of a known Congress leader to the effect that his brother was not well on purely humanitarian and compassionate grounds," Singh said in his letter.
Singh said he did not verify the application of Khan for issuance of Passport which contains a specific clause that the officer/MP verifying knows the person personally.
"As I did not know him hence I did not verify the application," he said.
The ED is also likely to quiz Khan's aide Kashinath Tapuria, who is in the agency's custody, after similar leads emerged in the case following the questioning of the politician.
Amlendu Pandey was grilled on April 14 by the ED for allegedly procuring passport from Patna for Ali by forging documents.
The ED had mentioned in its remand application that it was necessary to question him regarding transactions of Khan in Singapore and also to find out the reasons for obtaining the second passport from Patna.
Khan, Pandey and Tapuria had allegedly travelled to Singapore in 1997 to open an account in the United Bank of Switzerland.
Khan is accused of massive money-laundering, tax evasion and stashing away huge amounts in foreign banks.
According to a report of the Comptroller and Auditor General of India (CAG), Khan and Tapuriahs owe the government about Rs 75,000 crore in taxes.