The Basel Committee on Banking Supervision is considering extra capital requirements of as much as 3.5 percentage points that the largest banks may face if they grow bigger, according to two people familiar with the talks.

The so-called surcharge would take the form of a boost to capital the banks must hold and would apply to financial institutions whose collapse would harm the global economy. A list of such banks hasn't been disclosed.

Draft plans circulated before a meeting next week would subject banks to a sliding scale depending on their size and links to other lenders, said the people, who declined to be identified because the proposals aren't public. Banks wouldn't initially face the highest surcharge, which is intended as a deterrent to expansion, one person said. The largest banks may face a 3 percentage point levy at their current sizes, the person said.