While Mr Raynsford accepted that he had given up his role as minister of London in September, he stressed that he was continuing to be involved in the legislation which sets up the post of London mayor and the Greater London Authority because it was "very large and complex".
John Redwood, the opposition spokesman on the environment, transport and the regions, said there "could be a conflict between his two roles".
Mr Raynsford, speaking during the debate on the Lords amendments to the Bill, insisted: "This is a very large and complex Bill. For this reason and because of my considerable involvement in the earlier stages, I'm continuing to have a role in its passage through this House.
"But I haven't been responsible for decisions on policy on any of the amendments to the GLA Bill either here or in the Lords since I ceased to be minister for London.
"My involvement today is merely to bring my experience and knowledge of the Bill to the House's aid and assist the passage of this complex matter."
Mr Raynsford, a former mayoral hopeful, withdrew his campaign bid when Mr Dobson, the former secretary of state for health, announced that he was standing.
Eric Forth, the Tory MP for Bromley and Chislehurst, questioned whether it was "appropriate" for Mr Raynsford "to be present in the light of his very prominent political role involved in the mayoral elections to come.
"It may be that he would wish to consider whether it was appropriate and whether or not it might prejudice his position and the position of the candidate for whom he is giving such visible support."