LONDONLONDON (Reuters) - Short-sellers are sitting on estimated losses of $70.87 billion from their short positions in U.S. companies so far this year, data from financial data analytics firm Ortex showed on Thursday.

The hefty losses come as shares of highly-shorted GameStop jumped more than 1,000% in the past week without a clear business reason, forcing short-sellers to buy back into the stock to cover potential losses -- defined as a short-squeeze -- while retail investors then piled in to benefit from the surge.

Chasing shorted companies became a trend among retail traders, rippling across U.S. markets and Europe. Ortex data showed that as of Wednesday, there were loss-making short positions on more than 5,000 U.S. firms.

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Its data also showed that estimated losses from shorting GameStop at $1.03 billion year-to-date, while those shorting Bed, Bath & Beyond were looking at a $600 million loss.

Ortex said the figures are based on the change in trading prices between the start of January to Wednesday's close, and the number of short positions. The company sources short interest data from submissions by agent lenders, prime brokers, and broker-dealers.

(Reporting by Sujata Rao; editing by Thyagaraju Adinarayan)