It is alleged that after being paid £4000, he tabled five parliamentary questions that had been drafted by the bogus lobbyists, as well as a parliamentary motion. Parliamentary records show he asked questions this month about Fiji's suspension from the Commonwealth, and UK investment in its public transport.
In March he put down an Early Day Motion - a parliamentary device used to draw attention to issues - saying there was "no justification for Fiji's continued suspension from the Commonwealth".
Panorama released a clip appearing to show Mercer telling the undercover reporter: "I do not charge a great deal of money for these things. I would normally come out at £500 per half day, so £1000 a day."
Details of the allegations emerged hours after Mercer dramatically announced he would quit parliament in 2015.
"Panorama are planning to broadcast a programme alleging that I have broken parliamentary rules.
"I am taking legal advice about these allegations - and I have referred myself to the Parliamentary Commissioner for Standards.
A Tory spokesman said Prime Minister David Cameron thought the MP had "done the right thing".
- AAP