Cable & Wireless has until this afternoonto clinch the takeover of smaller rival Energis.
It has already threatened to walk away from the proposed deal if it is rejected by a rebel minority of Energis owners.
C&W said it would not " under any circumstances" return with an improved offer if the proposal does not receive the necessary backing.
It is thought negotiations were going on all weekend with those still holding out.
Investment banks with a quarter of Energis' debt have been playing brinkmanship - determined to squeeze a higher price out of C&W in the knowledge that the deal needs the support of 75 per cent of debt holders.
C&W confirmed its interest in buying last month, but has not revealed what price has been put forward to stakeholders. Its offer is thought to be worth £780 million.
The proposal will collapse if it does not get the three quarters majority by 5pm.
Analysts believe a takeover could create a telecoms company with UK revenues of more than £2 billion.
Privately-owned Energis, which offers products and services ranging from contact centres to data, voice and internet, has worked for corporate customers including the BBC, Thomas Cook, Wanadoo and Tesco.
C&W also provides services to business customers, but has recently set about building a new broadband arm following its acquisition of Bulldog.
The telecoms carrier has overhauled its operations in recent years, pulling out of the US market and heavily restructuring its UK business at a cost of thousands of jobs.
It returned to the black during the year to March 31, reporting profits of £363 million compared with a deficit of £224 million last time.