The share price of Vodafone has fallen below the threshold that allows Eircom to pull out of the deal to sell Eircell without incurring financial penalties.
Vodafone share prices have been trading at stg£2.45, but their average price is now down to stg£2.20.
It is now estimated that the value of the sale has fallen by over €1bn to €3.3bn.
The board of Eircom is still expected to recommend the sale of Eircell to its shareholders when they convene at the Point Theatre in Dublin on Friday.