Pipeline companies Williams Cos. Inc. and Energy Transfer Equity LP said on May 24 that a trial over tax issues that threaten their $20 billion merger is scheduled to begin on June 20 in Delaware.

The companies also set a shareholder vote on the deal for June 27, according to a filing with the Securities and Exchange Commission (SEC).

Williams has accused ETE of trying to get out of the deal and filed a lawsuit in the Delaware Court of Chancery to prevent the Dallas-based company from terminating its takeover over a tax issue or if the deal isn't closed by a June 28 deadline.

ETE said it has filed a counterclaim to Williams' lawsuit.

The lawsuits are the latest twist in what has been a testy transaction almost from the get-go.

Kelcy Warren, the billionaire CEO of Energy Transfer, set his sights on Williams last year to transform his business into one of the world's biggest pipeline networks, launching an unsolicited bid in June and reaching a deal in late September that was then worth $33 billion.

The timing was poor. Oil and gas prices dropped significantly after it was announced, the companies' shares dropped sharply and investors started to worry that the $6 billion cash portion of the deal would saddle ETE with too much debt.