Goldman Sachs and Morgan Stanley’s sometimes lucrative romance with metals, coal and oil could become prohibitively expensive under a rule proposed Friday by the Federal Reserve.

The long-awaited regulation would require banks to put up much more capital to support investments in physical commodities, restrict ownership of power plants and limit the amount of trading banks can do. While the Fed doesn’t have the power to sever banks’ ties to physical commodities, it is seeking massive capital increases for firms – especially Goldman Sachs and Morgan Stanley – that have been permitted to stay in those businesses based on special legal exemptions.

Fed officials said that the proposal would mean about $4 billion in additional capital for financial firms’ current activity.

“The proposal would help reduce the catastrophic, legal, reputational, and financial risks that physical commodity activities pose to financial holding companies,” the Fed said in a statement. What’s unstated is that the proposal also addresses years of criticism that banks could seize unfair advantages in metal and energy markets by owning hard assets and operating huge trading desks at the same time.

The proposal comes on the heels of a Fed recommendation made earlier this month that big banks should be barred from buying stakes in nonfinancial companies – a removal of merchant-banking abilities that Congress would have to initiate. Merchant banking will also face higher capital under the Fed’s proposal.

Together, the recommendations are part of the central bank’s broad goal to rein in how Wall Street invests its money outside of traditional lending. The effort stems in part from a 2014 Senate investigation that probed the industry’s sometimes controversial dealings in physical commodities, such as operating mines, warehousing aluminum and shipping oil.

That Senate probe accused Goldman Sachs, Morgan Stanley and JPMorgan Chase of using their ownership of metals and other physical commodities to dominate markets and gain unfair investing advantages. The physical commodities businesses at Goldman Sachs and Morgan Stanley were protected by grandfathering that allowed them wider abilities than most banks.