Reforming the UK’s membership of the EU will be a big, but not impossible, task, David Cameron is to say as he publishes a list of four demands later.
The letter to the president of the European Council will pave the way for detailed talks among EU leaders.
Mr Cameron has committed to negotiating a “better deal” with the EU ahead of an in/out referendum by the end of 2017.
Campaigners for Britain to leave the EU say talks on issues such as restricting EU migrants’ benefits are a gimmick.
Meanwhile, Europe minister David Lidington has said the government hopes to get a deal agreed as early as December, when a key summit of EU leaders takes place – but he cautioned that it was too early to guarantee exact timings.
In a speech ahead of the letter’s publication, Mr Cameron will say four objectives lie at the heart of the UK’s renegotiations:
- protection of the single market for Britain and other non-euro countries
- boosting competitiveness
- exempting Britain from “ever-closer union” and bolstering national parliaments
- restricting EU migrants’ access to in-work benefits such as tax credits.
BBC political correspondent Alex Forsyth said the issue of restricting benefits would be a “hard sell” as some countries feel that could discriminate against their citizens but it is an area the PM is committed to pursuing because it is the one that might address immigration.
Defence Secretary Michael Fallon told BBC Breakfast the negotiations would be “arduous, long but very substantial” and he believed the prime minister would be successful.
Mr Cameron has said he wants the UK to stay in a reformed EU, but he has not ruled out leaving if he cannot secure the change he wants with the leaders of the 27 EU countries.
His letter to Donald Tusk will be the first time he has set out in writing the details of what he wants to achieve.
Around 40% of EU migrants who have arrived in the UK over the last four years – about 224,000 people – are supported by the benefits system, new analysis from the Department for Work and Pensions suggests. Of these, around 66% – about 148,000 new arrivals – receive in-work benefits.
By BBC political correspondent Ben Wright
Today is a significant step towards the EU referendum, which must take place by the end of 2017.
The letter to the European Council is unlikely to be an itemised shopping list of demands. Instead it’s likely to set out the four – now familiar – objectives the government is pushing for.
The most politically-charged – and potentially hardest to achieve – is restricting in-work benefits to EU migrants for a number of years.
That was a Conservative Party manifesto pledge – and David Cameron is expected to repeat that commitment.
The Vote Leave campaign said it expected David Cameron to get what he’s asking for because it’s a “trivial” list of demands.
But as he lights the fuse for the next stage of intense, potentially difficult, negotiations to come, Mr Cameron will say the mission is big – but not impossible.
The prime minister insists he wants Britain to stay in a reformed EU. Getting a deal that matches the billing will be crucial for his case.
In his speech, Mr Cameron is expected to say: “When you look at the challenges facing European leaders today, the changes that Britain is seeking do not fall in the box marked ‘impossible’.
“They are eminently resolvable, with the requisite political will and political imagination.”
Alongside publication of the PM’s letter, Chancellor George Osborne will meet European Commission President Jean-Claude Juncker in Brussels as part of the renewed diplomatic push for EU reform.
The Vote Leave campaign said Mr Cameron’s negotiating demands were likely to be “trivial” and that the only way for the UK to regain control of its borders and democracy was by leaving the EU.
Labour’s shadow chancellor John McDonnell, meanwhile, has dismissed the PM’s position on the EU as “a lot of bluff and bluster” and more about “appeasing” some of his Eurosceptic backbenchers.
He told the BBC that Labour’s position was that Britain should stay in the EU and “negotiate our reform agenda as members of the club”.
Mr Cameron’s speech comes after he told the CBI conference on Monday he had no “emotional attachment” that would stop him backing a UK exit if his EU renegotiation failed.
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David Cameron is understood to want an early vote, but has already been forced to rule out holding the poll on 5 May 2016, the same day as national elections in Scotland, Wales and Northern Ireland.
There has been speculation the referendum could be held as soon as June next year – but Downing Street has dismissed this as “not true”.
In a speech on Monday evening, Europe minister David Lidington said the PM’s priority was the “substance” of the talks rather than timing.
Mr Lidington said: “He doesn’t want to hold up the discussions and agreement any longer than necessary but what he’s not going to do is to cut a deal which he is not satisfied with for the sake of a particular arbitrary deadline.”
But he added that there would be “a few months” between the date of the referendum being announced and the poll taking place.