A fresh bid to secure cross-party agreement on press regulation looked set to go "to the wire" as high-level political talks again failed to reach a deal.
Labour said it was united with the Liberal Democrats in refusing to accept Tory proposals for a watchdog set up by royal charter but not underpinned by law.
MPs are due to decide on Monday on the shape of a new system of self-regulation to meet the demands of the Leveson Report into phone-hacking and other abuses.
Prime Minister David Cameron - who pulled the plug on talks last week and said he would let MPs decide - faces the likely prospect of defeat in the Commons.
Discussions with Deputy Prime Minister Nick Clegg over a Lib/Lab alternative charter with statutory backing have resumed but Number 10 sources said no advance was expected tonight.
It came after Chancellor George Osborne and Culture Secretary Maria Miller indicated the door was open to a compromise - in an apparent bid to avert a damaging revolt.
A senior Labour source said: "We are in lock-step with the Liberal Democrats on this. Ed Miliband spoke to Nick Clegg twice before Nick spoke to David Cameron and once after. We are clear we are not going to accept their royal charter. Any agreement must be on the basis of our royal charter. We are planning to go ahead with the votes in the Commons."
The talks were focused on trying to "secure agreement on the royal charter plus that we published on Friday", according to a senior Liberal Democrat source. Differences between the charters are seen as surmountable issues surrounding the new watchdog's powers to enforce prominent apologies and the rights of the press to choose its membership.
The sticking point remains the proposals to enshrine the system in legislation - something the Prime Minister has warned threatens press freedom and could make the system unworkable.
However while Number 10 sources said the statutory underpinning was "unnecessary and undesirable", Mr Cameron signalled on Saturday that it was not "a big issue of principle". The Downing Street source said: "This will go to the wire."
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