The BBC Trust has warned the government it would fight any move to force the BBC to meet the cost of free television licences for the over-75s.
A BBC Trust spokeswoman said it would be "unacceptable" for licence fee payers to foot the bill.
Ministers are said to be considering transferring the cost to the BBC as part of the Spending Review.
But reports also say the BBC may be offered the alternative of footing the bill for the World Service.
The Department for Work and Pensions currently funds the £556m annual cost of the free TV licences for the over-75s.
That is almost exactly the cost of running BBC Two, according to the BBC's media correspondent, Torin Douglas.
The Foreign Office pays for the BBC World Service, currently £272m a year.
'Universal benefit'"Either scheme, if implemented in full, would add hundreds of millions of pounds to the BBC's costs, when it is under pressure to make substantial cuts in its existing costs," our correspondent said.
The BBC Trust spokeswoman, referring to the TV licences, said: "Anything at this stage is speculation as we have yet to see the detail of the Spending Review.
"That said it would be unacceptable for licence fee payers to pick up the bill for what is a Department for Work and Pensions universal benefit," she added.
According to Newsnight's political editor Michael Crick, if approved, the proposal would take effect from the time of the next licence fee settlement, due in 2012.
He added increases in the licence fee were unlikely to cover the cost, which was the equivalent of a 16% cut in the BBC's present budget.
A government spokeswoman said it would not comment on speculation ahead of the Spending Review.
Every pensioner over the age of 75 receives a free TV licence, which currently costs £145.50 per year.
The corporation received £3.45bn in licence fees in 2009/10