Prime Minister David Cameron said the allegations against Jack Straw and Malcolm Rifkind were "very serious matters" that should be investigated by Parliament.
Straw, foreign secretary under Labour Prime Minister Tony Blair, and Conservative former foreign secretary Rifkind were secretly filmed by reporters posing as representatives of a fictional Hong Kong-based communications agency allegedly seeking top U.K. politicians to join the firm's advisory board.
Rifkind was recorded as saying he could arrange "useful access" to ambassadors, while Straw spoke of using "charm and menace" to change politicians' minds.
As foreign secretary, Straw strongly backed the decision to invade Iraq in 2003, while Rifkind played a role in efforts to bring peace to the Balkans.
The sting by Channel 4's "Dispatches" program and the Daily Telegraph newspaper reopened a debate about political lobbying. Currently, legislators can have outside business interests as long as they declare them.
Labour Party leader Ed Miliband has called for lawmakers to be banned from serving as consultants or company directors.
Rifkind and Straw, who are still lawmakers, have referred themselves to parliament's standards watchdog. Both were suspended from their party caucuses.
Rifkind, who chairs the parliamentary committee that oversees the intelligence services, said allegations of wrongdoing were "unfounded" and defended lawmakers' right to make money.
"If you are trying to attract people of a business or a professional background to serve in the House of Commons ... it is quite unrealistic to believe they will go through their parliamentary career being able to simply accept a salary of 60,000 pounds ($92,000)," he told the BBC.
Legislators earn a basic salary of 67,000 pounds ($103,000).
Straw said his conversation related to activities he might undertake once he retires from Parliament in May. He said he had complied with the parliamentary ethics code and he had checked out the bogus firm before the meeting.
"My checks were not sufficient to overcome the skilful deception of the undercover reporters," he said.