Update: ... or maybe not, because shortly after S and TMUS tumbled, CNBC denied the original Nikkei report:

• SOFTBANK DOESN’T HAVE PLAN TO WITHDRAW FROM SPRINT TALKS: CNBC

It is unclear if this announcement was made as a favor to some of the deal's M&A arbs who are painfully blowing up today, but for what it's worth, it managed to send the stocks sharply higher.

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Sprint stock plunged, and was halted by the exchange volatility trigger, when the Nikkei reported moments ago that Japan's SoftBank Group plans to break off negotiations on the long-awaited merger between its subsidiary Sprint and T-Mobile US due to a failure to agree on ownership of the combined entity, "dashing the Japanese technology giant's hopes of reshaping the American wireless business."

According to The Nikkei, SoftBank is now expected to approach T-Mobile owner Deutsche Telekom as early as Tuesday to propose ending the negotiations. The pair had reached a broad agreement to integrate T-Mobile and Sprint - the third- and fourth-largest carriers in the U.S. - and were ironing out such details as the ownership ratio.

The German parent had insisted on a controlling stake, according to a source familiar with the situation. Some at SoftBank were initially amenable as long as the Japanese company retained some influence. But SoftBank's board affirmed at a meeting Friday that the company would not give up control. The decision was made Monday to call the talks off.

Meanwhile, in the latest nightmare announcement for M&A arbs, Sprint tumbled as much as 13% before resuming, with TMobile also dumping, as it now appears that this endless merger process is finally dead.