S&P Global Ratings says there's a one-in-three chance it will cut its assessment of government support of the major banks in a financial crisis, a move that would trigger a downgrade to their credit ratings and push up the cost of funding.

But for now, the international ratings agency says it believes both major political parties will "continue to hold a pragmatic view" that a taxpayer-funded bailout of any major banks would be more likely to maintain financial system and economic stability than forcing losses onto bond holders.