Andrew Tyrie MP, Chairman of the Treasury Committee, said it was 'concerning' to see MAS named by the Government yesterday.
The Pension Advisory Service was the other independent body listed to provide the guidance.
Mr Tyrie's committee has been severely critical of MAS over the last year and an independent review is on going.
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He has written to George Osborne to express his concerns.
Mr Tyrie said: "The Treasury Committee was clear that guidance provided under the guidance guarantee should be demonstrably impartial between insurance providers.
"Today's announcement by the Treasury that the guidance guarantee will be provided by independent bodies appears to meet this requirement, and is a step forward.
"It is concerning, however, that the MAS may play a part in the delivery of the guidance.
"The committee has expressed serious concerns about the ability of the MAS to perform its functions, and has recommended that an independent review consider whether the MAS should exist at all as a statutory body."
He has asked the Chancellor to clarify the scope and limits of the role proposed for the MAS.
He said: "Furthermore, the independent review should be allowed to consider both immediate improvements to the MAS and the question of its future as a statutory body.
"Without such an undertaking, the committee would have reservations about a role for the MAS in an area as important as the guidance guarantee."
Financial Planner Online asked MAS for a response to the criticism.
Jackie Spencer, retirement and pensions expert at MAS, said:
"The Treasury invited us along with other organisations to provide the guidance guarantee.
"We have been selected, and feel confident we are best placed to deliver this with TPAS."