The UK still has "issues" with the proposed EU reform package, Foreign Secretary Philip Hammond said after pro-Brexit campaigners said the latest draft watered down protections.
European Council president Donald Tusk has issued revised proposals to member states in response to concerns raised by other members of the 28-member bloc.
The new draft text being discussed in Brussels includes changes which would effectively limit the use of a proposed "emergency brake" on migrant workers in the UK and makes clear child benefit curbs will not be extended to old-age pensions.
Mr Tusk sent out the latest version after announcing he had cleared his schedule for talks with key EU leaders in a bid to secure agreement at a summit on February 18/19, admitting the negotiations remained "fragile".
Prime Minister David Cameron hopes to seal a final deal in Brussels, clearing the way for the promised in/out referendum before the summer.
Steve Baker, co-chair of the anti-EU Conservatives for Britain group, said the package was not only being watered down but now contained "hidden nasties that will be used to undermine British interests".
"A remain vote will be a green light for more money and power flowing to Brussels with Britain helpless to stop it," he said.
Asked if the UK could accept the package as it stands, Mr Hammond told reporters: "There is no deal at present; there is a working draft.
"We have issues, some of which have been addressed - language issues - in the latest iteration of the draft, some of which have not been addressed.
"So the discussions continue and I do not think it is sensible to draw any conclusions about the shape of the deal until we see the final text that emerges from the European Council meeting."
The most significant change in the new draft relates to the use of an "emergency brake" allowing countries to restrict EU migrants' access to benefits when the volume of incomers is causing excessive domestic strain.
It now says it is appropriate "in particular" for countries which did not take advantage of transitional arrangements to limit the flow of migrants from new EU members in eastern Europe in 2004 and 2007 - the UK, Ireland and Sweden.
The amendment appears to respond to concern from eastern European states that concession to the UK should not open the door for countries like Germany, Austria and Denmark to impose restrictions.
Shadow foreign secretary Hilary Benn issued an impassioned plea for the UK to vote to remain in the EU, saying the case was "stronger than ever" and that Brexit would be exploited as "weakness" by Russian president Vladimir Putin.
Delivering a speech at the Chatham House think-tank in London, Mr Benn said leaving would mean the UK was less able to deal with international challenges such as the migrant crisis and climate change.
Campaigners for Scottish independence would also take the opportunity to "have another go" at breaking up the union.
"Let's be clear. President Putin would shed no tears if Britain left the European Union," he said.
"He would see Brexit as a sign of our weakness and of the weakness of European solidarity at the very moment when we need to maintain our collective strength."
Mr Benn - who campaigned against British membership at the time of the last public vote in 1975 - said it had become clear over the subsequent decades that it was wrong to "close the doors and wish that the rest of the world would go away".
He insisted Labour leader Jeremy Corbyn - who left open his commitment to EU membership during the leadership campaign and has expressed concerns about the proposed transatlantic trade deal known as TTIP - was "fully in support of Britain remaining in the EU".