The ONS says it has decided to classify the banks as public entities from October 13, meaning their liabilities are included in public finances.
It could not confirm how much the banks will add to national debt, but the ONS estimates it is likely to add £1 trillion to £1.5 trillion to public-sector net debt.
The ONS says: “As a percentage of the gross domestic product, this range is roughly equivalent to between 70 per cent and 100 per cent of GDP.”
Liberal Democrat Shadow Chancellor, Vince Cable says: “The ONS has confirmed what we already knew. The Government has in effect nationalised RBS and has a substantial stake in Lloyds.
“However, it’s extraordinary that statistical conventions mean the public accounts will only include the liabilities of the banks, and not their assets.
“Ministers must now accept the reality that they effectively own two large banks and use them to ensure the flow of much needed credit into the economy.”