London(CNN) British Prime Minister David Cameron has welcomed a new plan outlining proposed changes to the terms of the UK's membership in the European Union, telling reporters that the document delivers "substantial change."
"At the beginning of this process, we set out the four areas where we wanted to see substantial change, and this document delivers that," he told reporters.
"But of course there's still detail to be worked on, there's important things to be secured, there's further work to be done."
The document, released by European Council President Donald Tusk on Tuesday, grants concessions in key areas being sought by Cameron, as he works to secure more favorable conditions of the country's EU membership ahead of a planned British referendum on whether to stay or leave the 28-member union.
Why you should care about Europe's new deal for the UK
Among the proposals are measures that would allow the UK to suspend welfare benefits to migrants from the EU for up to four years, and provisions whereby member states can block unwanted EU laws.
Other gains for Cameron include new rules to stop non-EU migrants from entering the UK through "sham marriages" to EU citizens and greater protection for countries, such as Britain, that do not use the euro as currency.
In a letter released alongside the document Tuesday, Tusk said that the plan "goes really far in addressing all the concerns raised by Prime Minister Cameron."
"The line I did not cross, however, were the principles on which the European project is founded," he wrote.
He believed the proposal was "a good basis for a compromise," he wrote.
In recent days, Cameron has been holding urgent talks with Tusk, who leads a body made up of the heads of the European Union's member states, to thrash out plans to reform the terms of the UK's EU membership in hopes of avoiding a potential "Brexit," or British exit, from the union.
Cameron has been seeking significant changes to the terms of Britain's European Union membership, in terms of social benefits and migration, sovereignty, economic governance and competitiveness.
The British referendum on whether to leave the EU will be held some time before the end of 2017, most likely this year.
Both sides have been seeking to reach an agreement so a proposal can be presented at a key summit next month before European leaders, who will need to grant their approval.
A key sticking point in discussions has been the eligibility of new migrants from the European Union to the UK to receive welfare benefits.
Cameron has sought rules preventing EU citizens working in Britain from claiming "in-work benefits" -- such as income supplements for those in low-paid work -- for four years, as part of efforts to reduce migration from the EU into the UK.
Other European countries have been opposed to this, saying it is discriminatory and runs counter to the EU's guiding principle of freedom of movement.
The draft deal proposes creating a so-called emergency brake mechanism that could be applied when countries experience an "inflow of workers from other Member States of an exceptional magnitude over an extended period of time."
But rather than a complete ban, it stipulates that access to benefits should be "graduated, from an initial complete exclusion but gradually increasing."
It would also reduce the benefits paid by the UK for children of migrant workers who remain in their home country, allowing them to be indexed to the standard of living in that country.
Among other key gains recognized in the document was a recognition that the UK "is not committed to further political integration into the European Union." Cameron has sought to end the country's commitment to an "ever closer union" with Europe as enshrined in EU treaties.
It also contained a provision for member states to show a so-called red card to to unwanted EU legislation. Under the proposal, if 55% of national parliaments in the EU, proportionally, objected to a law from Brussels within 12 weeks, their concerns would be heard.
The document also provided a guarantee that countries that did not use the euro as currency, like Britain, would not be called on to safeguard the financial stability of the eurozone.
The proposals also outlined new powers to stop suspected terrorists from entering the UK -- not only if a threat was "imminent" -- and guaranteed that the EU would continue efforts to cut bureaucracy.
Reaction to the document was mixed, with many eurosceptics -- those wary of European integration -- in Cameron's own party expressing their disappointment.
Liam Fox, a Conservative MP who is in favor of a Brexit, said that the concessions fell short of what Cameron had promised.
"The very limited set of demands from our government have been watered down by the EU in every area," he said.
"None of these changes even come close to the fundamental changes promised to the public," he said. "We are being asked to risk staying in the EU based on the back of empty promises from the EU that are not even backed up in Treaty."
Arron Banks, head of the eurosceptic Leave.EU lobby group, released a statement labeling the proposals a "worthless package of so-called reforms."
"The idea that an emergency brake on benefits will do anything to reduce immigration ... would be laughable enough even if we didn't have to get Brussels' permission to pull it," he said. "Controlling a brake is a pretty poor second to having your hands on the steering wheel."
But the proposals won some endorsement in the UK and in Europe, with Danish Prime Minister Lars Lokke Rasmussen tweeting that the document was a "good basis for negotiations."
Three years ago, Cameron pledged to put Britain's membership of the EU to a public vote if his Conservative Party won the 2015 elections, which it did.
A key element of his proposal was to seek changes to the rules concerning the UK's membership of the union, before the issue was put to a public vote.
Cameron says he wants the UK to remain in the union, but under reformed conditions.
Polls suggest that British public opinion is closely divided on the Brexit question. The latest YouGov poll of 2,438 adults said that 42% favored leaving the EU, with 38% wanting to remain.
A poll of polls averaging the results of the last six polls on the issue has found 53% in favor of remaining and 47% in favor of a Brexit.