Last year Mercer referred himself to the Parliamentary Commissioner for Standards, as well as quitting the Tory whip, after a joint investigation by The Daily Telegraph and Panorama.
Undercover filming showed him apparently accepting £4,000 to lobby on behalf of Fiji, a country that has been accused of human rights violations.
He told the journalists posing as lobbyists that he would set up a parliamentary group to press for the nation’s return to the Commonwealth. He also delivered on a promise to table questions in the Commons on the issue.
In a statement outside the Commons yesterday ahead of the expected publication of the commissioner's report on his conduct today, Mercer was reported as saying: "It is clear to me the constituency of Newark needs to be represented properly. I will not argue with the findings of the committee and I can only humbly apologise.
"My military background has taught me that you must never let down the people who depend on you, and those people are my constituents of Newark. If that becomes the case, you must go without any argument."
Mercer was not the only political figure to fall foul of media stings over lobbying. Lord Laird, who has denied wrongdoing, was kicked out of the CIPR earlier this year after being suspended from the House of Lords for six months.
In the wake of Mercer's announcement there was speculation that UKIP leader Nigel Farage would contest the seat but Farage said this morning he had decided not to run.
He told the media that he did not want to look like an "opportunist" by running for the seat after Mercer's departure.