The ruling by the Speaker came in response to a reference from JPC chairman PC Chacko on whether there was a conflict of interest if MPs who were ministers in governments in the period under probe between 1998 and 2009 were members of the panel.
Parliament sources said that Chacko's query was a broad one and did not mention any name.
Singh and Sinha were ministers in the BJP-led NDA governments during the period. At the JPC's first meeting on March 24, Congress, Trinamool Congress and the Left parties had raised questions on their presence in the panel.
The sources said that the Speaker has cited the relevant rules of procedure and conduct of business in Lok Sabha in her ruling to conclude that unless a specific objection is pinpointed, a general query on members having been ministers in the past cannot be a ground for debarring them from committees.
Congress' opposition to the Public Accounts Committee headed by BJP's Murli Manohar Joshi continuing with the probe into the 2G scam had also not found favour with the Speaker, who had asked both the committees to work in close cooperation and harmony.
Sinha had held the finance portfolio between 1998 and 2003, while Singh was the foreign minister and headed the GoM that dealt with the telecom policy. Later, both swapped their portfolios and the kept their ministerial berths till the NDA government was voted out in 2004.
The terms of reference of the JPC include examining "the telecom policy prescriptions by successive governments, including decisions of the Union Cabinet in the allocation and pricing of telecom licences and spectrum from 1998 to 2009.
Meanwhile, a Congress member of the JPC said that the report of the PAC, which is expected to be available soon, was likely to go into it by the JPC.