If Nippon Sheet Glass does not return with a serious, improved offer soon, discussions will be terminated imminently, analysts say.
NSG's management returned home almost a month ago after a series of meetings with Pilkington's board to conduct "high-level" due diligence. Since then, the advisers on both sides have been infuriated by NSG's reluctance to play their cards.
In early November, Pilkington rejected a £2billion, 150p-a-share offer for falling "materially short of a price [it] is willing to recommend". Bankers and analysts had expected NSG to return with an offer of around 165p a share by now.
Instead, Pilkington's shares have slowly edged lower, from 155p after the approach to 149¾p yesterday, as the prospects of a deal have grown more remote.
NSG's approach was always viewed as opportunistic. The Japanese glassmaker, number two in its own market, is about half the size of Pilkington, but has the backing of Sumitomo Bank. It also owns 20pc of Pilkington, a legacy of the world's second largest glassmaker's problems in the 1990s.
Analysts fear that, if talks come to an end - which may happen this week - the shares will tumble. The stock jumped 21p on the news of a bid. Paul Roger, of Deutsche Bank, said: "The downside if the deal collapses is substantial."