Sky formed a committee of independent directors to consider the bids, given James Murdoch's role as chief executive of Fox.
"A further announcement will be made in due course", it said.
Comcast is considering whether to urge investors to go with its offer instead, The Wall Street Journal reported earlier Wednesday. The stock has been trading above the Comcast offer price since the company indicated its interest in late February.
In withdrawing its recommendation and killing the co-operation agreement, the committee added that Fox would no longer be on the hook for provisions including a weighty break fee that could have totaled £200 million ($278 million).
Comcast, the US-based media group, is aiming for the winning bid in the sale of Sky a European media and telecommunications broadcaster based in the UK.
"We think (Comcast) could reasonably take the offer up to 1300p, but they would be giving away value at anything above that", New Street Research analyst Jonathan Chaplin said.
NBCUniversal's parent company confirmed that its all-cash offer was, as previously signaled, for £12.50 per share, which is a 16 percent premium to Fox's existing bid of £10.75 per share.
And in a bid to head off any competition concerns, the group said it would vow not to acquire any majority stake in any United Kingdom newspapers for five years. Sky has a strong business, excellent customer loyalty, and a valued brand.
After Comcast's formal bid, Sky withdrew its recommendation that shareholders should accept Fox's offer.
That undertaking was anticipated to ameliorate any concerns over plurality, although the Fox-Disney deal remains under the scrutiny of USA regulators.
Comcast pledged to keep several of Sky's assets like Sky News and maintain its corporate headquarters in the UK. Nevertheless, investors who pushed Sky shares up 4 percent to £13.57 on Wednesday are wagering that Disney and Fox will disregard financial sense.
Comcast sees Sky's 23M customers in the UK, Italy and Germany as a platform for further growth on the Continent.