Lord Mandelson has taken up a new role advising the oil giant BP, just months after criticising Ed Miliband's plan to cap electricity bills.
BP confirmed that the former business secretary was providing advice, despite the Labour peer's continued refusal to reveal the names of the clients of his Global Counsel advisory firm.
The oil giant told the Daily Mail: "Global Counsel have done some work for BP, for instance on the Southern Corridor [the gas line project from the Caspian]."
The House of Lords passed new rules in 2012 that required peers to set out who they advised in order to avoid any suggestion of conflicts of interest, however Lord Mandelson has avoided to reveal his by exploiting a loophole in the rules.
Lord Mandelson previously said Labour policy was being "taken backwards" under Ed Miliband's leadership.
"At the business department I tried to move on from the conventional choice in industrial policy between state control and laissez-faire. The industrial activism I developed showed that intervention in the economy - government doing some of the pump priming of important markets, sectors and technologies - was a sensible approach."
After the Labour peer's intervention into the energy bills debate, Labour MP Tom Watson urged the peer to reveal his connections, warning that: "Lord Mandelson has become a lobbyist now and if he wants to pass comment, he can."