Labour has asked the Cabinet secretary to investigate whether David Cameron may have broken the ministerial code over his relationship with Lynton Crosby.
The letter comes amid a growing row about whether the election strategist, who is also a lobbyist for a large tobacco firm, had an influence on the coalition's decision to drop plans for plain cigarette packets.
Cameron was repeatedly asked about whether he had ever had a conversation with Crosby, who works one day a week for the Tories, about plain packs during PMQs today, but he appeared to evade the question.
As in previous instances the prime minister said he had never been "lobbied" by Crosby, but would not answer whether they ever discussed the issue in a less formal capacity.
"He is the prime minister for Benson and hedge funds and he knows it," Miliband replied.
"It's Andy Coulson all over again. He's a PM who doesn't think the rules apply to him. He always stands up for the wrong people."
Cameron, who had a good day at the despatch box and was widely considered to have won the clash, replied: "The reason his leadership is in crisis is he can't speak on the big issues. Every day this country is getting stronger and every day he is getting weaker."
Plans to introduce plain packs had considerable momentum behind them but were postponed indefinitely while ministers observed what the effect of a similar policy was in Australia.
The sudden about-turn surprised observers and raised concerns that the decision may be linked to Crosby's role in the Tory party.
Crosby is a controversial Australian election strategist who worked on John Howard's campaign in his home country and helped Boris Johnson into City Hall.
He is known for his shouting matches with colleagues, iron discipline and promotion of solidly right-wing messages on topics like welfare and immigration.