M&G plans to talk to a mixture of advertising and direct marketing agencies about the account and has appointed Agency Insight to conduct the review.
HTW has been invited to repitch for the business.
M&G's former marketing services director John Goddard hired HTW in 2001.
The agency's first task was to sell M&G's ISAs.
The agency produced a series of long-copy ads explaining why people should take out M&G ISA products. The ads scooped three golds at the 2002 Direct Marketing Awards.
The agency also developed a print campaign featuring illustrations of Hattie Jacques and Leonard Rossiter to promote its Recovery and Dividend Funds.
Last year, M&G spent £1.44 million across all its direct, press and outdoor advertising, according to Nielsen Media Research.
The review is being overseen by Jackie Haskins, who replaced Goddard as M&G's marketing services director last May. Goddard had left to join the Abbey-owned online bank Cahoot, where he is the managing director.
In a statement, M&G said the review had been instigated because "it is good practice to review agencies from time to time".
M&G's media account, held by Zed, is being pitched separately as part of a group review by M&G's parent, Prudential, which also owns Egg.