The amount of investors' assets held in the worst-performing funds has risen 68 percent to £22.32 billion ($36.9 billion) in just six months, according to investment adviser BestInvest.
M&G, one of the U.K.'s biggest fund managers, suffered a blow as three of its funds slipped into the 'dog' category in BestInvest's biannual Spot the Dog ratings. The league table of UK-based fund managers, lists funds that have underperformed their benchmark by 10 percent or more over three years. The last time the research was carried out, M&G had no "dog" funds.
The M&G Recovery fund, one of its best-known funds with £7 billion under management, and led by Tom Dobell, has been particularly badly hit, with BP shares a huge drag on its performance after the continued fallout from oil giant's Deepwater Horizon leak. The fund, which also owns a substantial stake in budget airline Easyjet, underperformed its peer group by almost 20 percent over the period.
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Dobell, who is focused on long-term investment, has asked for his investors' patience with the fund's performance. M&G's Global Basics and American funds also fell into the "dog" category.
M&G had not responded to a request for comment by the time of publication.
Fellow large fund managers F&C and Scottish Widows Investment Partnership (SWIP) also had three "dog" funds.
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"All funds and managers go through periods of difficult performance during their careers, so we are not saying investors should automatically switch out of these funds," Jason Hollands, managing director at Bestinvest, said.
"However, if you hold any dogs, you certainly need to explore further whether you should continue to give it more time to recover or whether it may be better to move elsewhere."
- By CNBC's Catherine Boyle. Twitter: @cboylecnbc.