David Cameron has received Denmark's backing for his EU renegotiation after Poland said plans to limit benefits for migrants need "further discussion".
Danish PM Lars Lokke Rasmussen said plans for a brake on benefit payments were "understandable and acceptable" and he would be as "supportive as possible" to keep the UK in the EU.
Mr Cameron is trying to garner support for an EU-wide deal in two weeks' time.
He insisted any agreement would be "legally binding and irreversible".
However, European Parliament president Martin Schulz has said he could not guarantee MEPs would "adopt" the welfare proposals without amendment and said they would be subject to months of debate, potentially delaying their implementation.
Mr Cameron, who has spent the day in Poland and Denmark, has said he is making progress towards his goal of rewriting the terms of the UK's membership of the EU.
He needs the backing of all 28 EU leaders for his draft renegotiation package and hopes to secure it at a summit on 18 and 19 February, thus paving the way for a UK referendum in June.
Following talks in Copenhagen, Mr Rasmussen said Denmark - a country historically sceptical about the EU and which also retains its own currency - did not object to any of the UK's proposals and did not expect them to be significantly amended in the coming weeks.
He said the EU needed to retain a "strong British voice". "I truly believe that adopting this package will create a better Europe," he said.
On the issue of welfare curbs, which is proving a stick point for a number of other EU members, Mr Rasmussen said individual members should be able to "protect" their national welfare systems from abuse.
"It creates momentum towards the goal of ensuring that the EU does not develop into a social union," he said.
"We need to ensure that EU citizens move across borders to work, not to seek a high level of benefits.
"The package also contains an emergency brake which is specifically designed to handle the particular problems faced by the UK in regard to in-work benefits. That is perfectly understandable and acceptable to us."
Poland and other members of the so-called Visegrad group - the Czech Republic, Hungary and Slovakia - have expressed about the welfare restrictions, warning that it breaks freedom of movement rules and discriminates against their citizens.
Analysis by the BBC's Ben Wright, in Copenhagen
Poland could have been a difficult country for David Cameron to win over.
With a million Poles working in the UK the prospect of new welfare curbs on EU workers has been particularly contentious in Warsaw.
Politicians there have consistently warned against any measure that would cause discrimination.
But now, on the table, is a proposal Poland seems set to support.
After meeting David Cameron this morning, the leader of the governing Law and Justice party, Jaroslaw Kaczynski, said he was "satisfied" with the plans to reduce in-work benefits for four years and stressed the measure wouldn't affect Poles already living in Britain.
Mr Kaczynski is a power-broker in Polish politics and said his country had gained "very much" from the negotiations, including the right of Polish workers in the UK to continue sending child benefit back home, albeit at a reduced rate.
Speaking after a meeting with the Danish prime minister in Copenhagen, a buoyant David Cameron sounded optimistic about sewing up a deal.
After talks with Mr Cameron, Polish PM Beata Szydlo said she backed the UK's plans to boost national sovereignty and raise competitiveness but said the welfare needed to be "ironed out" to ensure Poles in the UK were not disadvantaged.
"There are always topics that need to be ironed out," she told reporters. "Over a million Poles live and work in Britain. Their work is growing Britain's GDP and we want them to enjoy the same kind of opportunities for development as Britons."
Mr Cameron said the talks were "very good" but acknowledged there was "important detail to be filled in".
He stressed he was working towards a "full strategic partnership" between the two countries on a range of issues, including that of security, amid Polish concerns about the growth of Russian influence on its eastern flank and what Nato's response will be.
Countries on the EU's eastern flank are to meet next week before providing a joint response to the reform package, Hungary's foreign minister said.
The draft proposals were published by European Council president Donald Tusk on Tuesday after months of negotiations between UK and EU officials and pave the way for Britain's EU referendum to take place as early as June.
An EU source close to the negotiations told the BBC the proposals had met initial resistance from many countries and Mr Cameron faced two weeks of "difficult" negotiations before the summit in Brussels.
Further reading on the UK's EU referendum
Referendum timeline: What will happen when?
The view from Europe: What's in it for the others?