US telecoms company Verizon has threatened to pull out of its $7.6bn (£4.01bn) deal to buy MCI.
Both Verizon and Qwest have increased their initial offers for MCI
The company said it would end the deal if MCI's board deemed a rival $9.06bn bid by Qwest Communications "superior".
In a letter to MCI, Verizon said Qwest had made an inferior offer and voiced fears that the interests of short term investors were driving the decision.
The news came after MCI reopened talks with Qwest, despite reaffirming its acceptance of a bid from Verizon.
Verizon had given MCI the go-ahead for the renewed talks with Qwest.
The long-distance phone company has twice given Verizon's offer the nod, on the grounds that the larger company offered better prospects.
But Qwest has given the firm until Tuesday to reconsider its latest bid.
Several MCI shareholders have been urging the firm's board to reconsider the offer claiming that Verizon's price is too low.
MCI had cited Verizon's robust financial health and strong growth prospects as the reasons behind it accepting the bid.
Verizon underlined its security in its letter of complaint saying it was concerned that the company's "long-term strength and viability" were being ignored in the decision making process.
"Should this occur, we would no longer be interested in participating in such a process," it added.