May 13 (Reuters) - Williams Companies Inc said it has filed a lawsuit against Energy Transfer Equity LP to prevent it from terminating its once-coveted deal for Williams.

Under the lawsuit filed in the Delaware Court of Chancery, Williams seeks to stop ETE from relying on any failure to close the deal by the June 28 deadline.

The latest development comes after Energy Transfer said in April its lawyers may not be able to deliver an important tax opinion for its takeover of Williams, throwing the agreed acquisition into doubt.

The lawsuit seeks to also prevent the pipeline company from avoiding its obligations in the event it fails to obtain the tax opinion.

Energy Transfer chief executive Kelcy Warren, a Dallas billionaire, set his sights on Williams last year to transform his empire into one of the biggest pipeline networks in the world. However, a prolonged drop in oil and gas prices has made the deal less economically attractive.

Williams' board continues to recommend the merger announced in September last year, the company said on Friday.

Tulsa, Oklahoma-based Williams had also filed a suit earlier against ETE in Delaware to stop a controversial offering of preferred shares to its top shareholders. It has also sued Energy Transfer's Warren in Texas over the same offering.

Williams has alleged that ETE is looking into ways to walk away from the tie-up even though the terms of the deal would not allow that.

Energy Transfer Equity was not immediately available for comment outside regular U.S. business hours.

(Reporting by Aurindom Mukherjee in Bengaluru; Editing by Richard Borsuk)