A leading analyst has claimed any investor considering buying into Groupon, the loss-making discounts website preparing to float in the US, is a "fool".

Sucharita Mulpuru, an analyst at Forrester Research, weighed into the debate over Groupon by warning that there is "no rational math that could possibly" suggest the company is worth the valuation that its planned flotation suggests.

Last week Groupon said it intends to raise $750m (£460m) on the US markets at a price that could value the two-and-half year old company at more than $20bn.

"There is no rational math that could possibly get anyone to the valuation Groupon thinks it deserves," the analyst said in an open letter to potential investors. "This IPO game isn't about finding value, it's about finding a greater fool who actually believes the valuation is true. Trust me, you will be the fool."

Mulpuru estimated Groupon's true value, based on financial information released with its flotation proposals, at closer to $2bn.

According to the filing, Groupon's revenues increased by 223% to $713.4m, but Mulpuru said $615m of that came from acquisitions and heavy marketing spending. Mulpuru also said Groupon spent $250m on marketing, which works out at $31 per new customer. Only $96m of last year's revenue was "truly organic growth", she argued.

Andrew Mason, the 30-year-old founder of Groupon, has said the IPO process will be a "bumpy ride". He insists, though, that Groupon's model of offering group discounts to potential customers over the internet will radically reshape the way people shop.

Last December Mason rejected a $6bn offer for Groupon from Google.

• Read an exclusive interview with Andrew Mason in the Guardian on Saturday