WorldCom has also warned that further charges are to come.
The company unveiled the exceptional item, which is believed to be one of the largest ever taken by a company, in filings with the US Securities and Exchange Commission on Thursday evening.
The charge is expected to be related to MCI's research and development projects which are already in progress.
WorldCom said it was conducting a valuation study of MCI assets in order to allocate a price to net assets being acquired in the deal.
Meanwhile, WorldCom said it was also drawing up plans to integrate MCI into its own operations - a move that was expected to lead to further charges in relation to "exit and restructuring costs".
However, WorldCom refused to specify or indicate the size of the additional charge other than to say that it "may be significant" and that it would be written off against profits when it was incurred.
The filings did not ruffle US investors, who pushed WorldCom shares up 44 cents to $53.06 yesterday. The shares have risen by more than 75 per cent since the beginning of the year.
The WorldCom-MCI merger, which was announced last year, is expected finally to be completed towards the end of this month when it receives the stamp of approval from US telecoms regulators.
The European Commission and the US Department of Justice have already given the deal the green light after MCI agreed to sell its Internet assets to Cable & Wireless, the UK group, for pounds 1.75bn.
WorldCom has negotiated a $12bn credit line as part of its financial restructuring following the merger and Internet sale deals. On Thursday, the group launched a $6.1bn bond issue, the largest ever by a corporation.
WorldCom is due to pay $7bn in cash to British Telecom in return for the 20 per cent shareholding the UK group holds in MCI.