As the BBC Trust met on Wednesday to discuss the proposal, the MPs warned it could be the thin end of the wedge, which paved the way for ads across the BBC's services.
In a Commons motion, they said the move risked undermining the BBC's impartial reputation. It has been signed by 31 Labour MPs, seven Liberal Democrats, three Democratic Unionists, two Tories, one Plaid Cymru MP and one Scottish Nationalist.
The MPs said the plan could throw away the BBC's hard-won reputation for just a few million pounds. They asked: "How long before BBC management contemplates their (ads) appearance on the World Service, on web pages accessed within Britain, on Radio Three, on CBeebies?"
The MPs said the Trust, which replaced the board of governors, had a duty to safeguard the public interest, which meant keeping the websites "commercial-free".
- Comment, page 52.