Forget subprime mortgages — one of Wall Street’s biggest risks doesn’t even show up on most banks’ balance sheets.

Financial insiders are getting increasingly worried over the popularity of securities-based loans, or SBLs — a risky form of debt marketed to wealthy investors who typically use it to buy big assets like houses.

The loans, which are taken against pools of stocks and bonds, offer borrowers cheap money fast without having to sell their underlying securities — an attractive option when the Dow is rising.

But if markets crash, brokers can unload their clients’ holdings at fire-sale prices — and go after the house to cover the the vig.

Fears of such ugly scenarios are growing as the Fed hikes interest rates, stocks are hitting all-time highs, and high-net-worth individuals are using this form of “shadow margin” to borrow more against stocks and bonds in their portfolios than ever before.

It’s not clear how much debt has been taken out in the form of SBLs, and a lack of regulatory oversight is partly to blame. Finra, the brokerage regulator, doesn’t track it, nor does the Securities and Exchange Commission — even though both have warned investors about the risks.

However, several advisers surveyed by The Post estimated there is between $100 billion and $250 billion in outstanding SBLs among all brokerages.

At least one concerned financial executive is in talks with lawyers to file a whistleblower case over the issue against a major bank with the Securities and Exchange Commission, The Post has learned.

“When the market does turn, and it will at some point, it will be a major disaster,” said the exec, who requested confidentiality in exchange for speaking on the issue with The Post.

Fans of SBLs have promoted them as a win-win on Wall Street. Brokers like them because they collect fees on them, unlike when they extend margin on stocks. They can also use terms of the loans to keep clients from fleeing to other money managers. Borrowers, meanwhile, get money at rates as low as 3.5 percent, depending on how rich they are.

According to prospectuses seen by The Post, SBLs have other borrower-friendly features. Unlike most other loans, SBLs typically have no term limits and don’t require monthly payments, even as interest compounds. Because the bank can sell off their securities at any time, borrowers aren’t required to chip away at their debt.

There are also some restrictions on the loans. Clients aren’t supposed to use them to buy other stocks or pay off outstanding margin, for instance. Still, there are no immediate repercussions for disobeying the rules, and regulators aren’t watching, sources said.

“What they’ll do is have another account, wire the money out of [the SBL account] to a bank, and then have that bank wire to pay off margin. I have seen that,” one ex-Morgan Stanley broker told The Post.

“You’re basically robbing Peter to pay Paul.”

Morgan Stanley didn’t immediately respond to requests for comment.

“Not all [SBLs] are ‘shadow margin,’ but certainly it’s been the case in a lot of the situations I’ve been involved in,” Louis Straney, a managing partner at Arbitration Insight, and an expert witness for trials involving SBLs, told The Post.

SBLs are a huge money-maker for brokerages, and were at the center of a years-long conflict-of-interest scandal at Morgan Stanley, which settled a lawsuit with Massachusetts last week over questionable sales contests that pushed the debt.

Morgan Stanley is one of the few firms that says how much in SBLs it’s sold — $36 billion, as of Dec. 31, a 26-percent increase from the year before.

Other major sellers of the loans are UBS, Bank of America, Wells Fargo, Raymond James, and Stifel Nicolaus, sources said.