Google Finance, the search giant's website that provides "real-time stock quotes, charts and financial news" listed Royal Mail's closing share price on Tuesday at 330 pence. This is despite the actual price soaring.
The service, that allows investors to track their portfolios, appeared to be showing the value at which the government initially priced the shares.
Shares in the postal firm have risen sharply and closed at 489p on the company's first day of unconditional trading in London, up almost 3%.
Google did not respond to requests for comment.
However a disclaimer on the website says the company is not liable for any errors and that "Google does not verify any data and disclaims any obligation to do so."
The sharp rise in the Royal Mail share price has led to criticism that they were sold too cheaply by the government.
Business Secretary Vince Cable and investment bank Lazards face fresh questions from MPs over the privatisation amid concerns that ministers heavily underpriced the shares.
Cable dismissed the steep rise when conditional trading began last week as "froth" but days later the stock was still ticking up, peaking at 490.7p yesterday.
Its closing price of 489p implied a rise in the company's value of £140 million in one day, to reach £4.9 billion.
The initial offer had valued Royal Mail at £3.3 billion.
The newly-privatised company faces the prospect of a national strike by more than 100,000 postal workers due to the continuing controversy.