Spark has lobbed an extra obstacle in the way of Sky's proposed merger with Vodafone.

Spark would go to court over Sky Television's proposed merger with Vodafone.

Sky Television says Spark, Trustpower and InternetNZ had advised Sky and Vodafone that they would go to court if the Commerce Commission approves Sky's planned merger with Vodafone.

The Commerce Commission is due to rule on the merger next week.

Sky said it and Vodafone had received letters from lawyers representing Spark, Trustpower and InternetNZ seeking assurances that Sky and Vodafone would not complete the merger, were they to get approval, "for a period of time that will enable the parties to challenge the Commerce Commission decision in court".

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Sky said it and Vodafone had declined to provide the requested assurances.

"Counsel for Spark indicated in their letter that if the assurances are not provided they anticipate being instructed to seek an interim stay. Sky does not consider that there is any proper basis for seeking an interim stay from the courts and, should any party seek an interim stay, Sky intends to oppose any such application and seek an undertaking as to damages," it said in a statement to the NZX.

"In the event the Commerce Commission provides clearance in respect of the proposed merger, Sky intends to proceed to completion in an orderly manner upon satisfaction or waiver of any outstanding conditions," it added.

When Sky and Vodafone first announced their proposed merger, Spark was quick to downplay the implications, saying in a media release that it didn't believe it would pose a greater challenge to Spark than the existing partnership between Sky and Vodafone.

"From a competitive perspective, Spark competes hard with Vodafone NZ every day. But we don't really see ourselves as competing head-to-head with Sky TV. The real competition in the future of media is with global over-the-top players like Netflix, YouTube and Apple or with direct-to-consumer premium sports content owners," it said in a statement in June.

However, Spark did an about-face the following month when it formally opposed the merger and later said Sky Sports was a "must have" for a "substantial number of Spark customers".

Spark has said that if the merger went ahead, Sky/Vodafone would have an incentive to use its control of premium sports to force more consumers to buy broadband or mobile services from Vodafone.

The Commerce Commission took the unprecedented step of proactively releasing a "letter of unresolved issues" in October setting out its concerns with the merger.

Sky spokeswoman Kirsty Way said Spark had had nine months to take action and it viewed the new legal threat as a "distraction".

"This late move indicates to me that [Spark chief executive Simon] Moutter is more concerned about our merger than the statements he made last year. Coupled with his tweets about sending back his Sky box, while still enjoying Sky it turned out – I'm not sure we can take his announcements at face value," she said.

Spark spokesman Andrew Pirie said Spark was only asking for a couple of days during which the merger would not be put in stone.

If the merger ruling went against Sky and Vodafone, they would have 20 days to appeal. But if it went in their favour, Spark would need to seek a judicial review, which could be stymied if the merger had already been enacted, he said.

He expected Spark would announce on Friday or Monday whether it would seek an interim stay.

The move was not based on any changed perception for Spark of the chances of the merger getting Commerce Commission approval, Pirie said, nor did he expect the commission to be swayed in its ruling by the possibility of legal action if it approved the merger.

Sky TV shares were trading down 2.2 per cent at $4.49 in late afternoon trading. Spark shares were down 3.6 per cent at $3.57.