General Electric shares tumbled after a major credit rating agency echoed concerns about the business raised last week by Bernie Madoff whistleblower Harry Markopolos.

Fitch Ratings issued an insurance industry update that examined 16 long-term-care insurance providers — and designated GE’s portfolio the second-riskiest. The only long-term-care insurer deemed riskier was Genworth Financial, which was spun out of GE in 2004.

Fitch called GE’s insurance business “below average” in terms of reserves, and said its exposure to costly long-term-care insurance payouts is “very high.”

The comments come just days after Markopolos on Thursday called the lightbulb maker “a bigger fraud than Enron” — sending the stock down 11%. Like Fitch, Markopolos also targeted GE’s long-term-care insurance business, saying losses there could force GE to file for bankruptcy.

GE shares slid 3.3%, to $8.38.

With over 250,000 long-term-care insurance policies and only $21 billion in gross reserves, GE is one of a handful of US insurers stuck with decades-old care commitments that have proved much more expensive to service than they were when sold, explained Anthony Beato, Fitch’s director of insurance.

The market for long-term care, which covers assisted living and nursing home expenses, “remains a plague” for insurers, Beato added.

GE countered that it is not under-reserved compared to its peers.

“Our current reserves are well-supported for our long-term care portfolio characteristics,” it wrote to The Post. “Our future liabilities depend on variables that will play out over decades, not years.”

The conglomerate also received an endorsement from Goldman Sachs analyst Joe Ritchie, who, after reviewing “reserves per policy” across the industry, said in a report that GE seemed better positioned than many of its peers.