Arbitrageurs had bought MCI shares in the hope of making a profit when the deal was completed, because the American group's stock was trading well below the value suggested by BT's original takeover offer.
Accounts filed by Mr Soros recently showed he increased his holding of MCI shares from 6 million to 13.3 million between April and June, giving him 2.4 per cent of the company. The 15 per cent fall in MCI's share price yesterday suggested Mr Soros's stake had fallen in value at one stage by around $70m.
BT has offered a mixture of $6 in cash plus 5.4 BT shares for every MCI share, which meant as the UK group's share price rose, so did the value of its takeover bid.
Yesterday's closing BT share price of 412.5p valued each MCI share at $41.5, compared with the trading price of just over $31.
The difference between the two prices reflected the fact that analysts and investors now expect the deal to be renegotiated at a lower price or abandoned altogether.