Vocus Communications and Amcom Telecommunications are preparing to fight TPG Telecom "to the very last shareholder" in a desperate attempt to stop their $1.2 billion merger from collapsing.

As exclusively revealed by Street Talk Online, TPG on Wednesday night raised its stake in Amcom from 6.7 per cent to a 16.7 per cent blocking stake - just one week before the merger was set to be voted on.

Vocus chief executive James Spenceley (left) with chairman David Spence (centre) and Amcom chairman Tony Grist (right) have their work cut out trying to salvage their planned merger. Credit:Peter Braig

The stake may be large enough to stop the merger because 75 per cent of the voting shareholder base must vote in favour of the deal for it to pass and Vocus is not allowed to use its 10 per cent stake in Amcom to support its own bid.

This means that between 80 and 90 per cent of Amcom shareholders must turn up and support the merger for it to succeed - a high percentage for non-compulsory votes.