David Cameron will not be able to deliver fundamental reform of the EU without changing its treaties, a committee of MPs says.
The
European Scrutiny Committee said there was not enough time for this to
happen ahead of the UK's in-out referendum, promised by 2018.Mr Cameron will try to persuade other EU leaders to back his reform plans at a summit on Thursday.
The government said it wanted "legally binding, irreversible changes".
Where changes to the EU's governing treaties are required, it said it would demand "agreement to such treaty change" before the referendum.
But Conservative Eurosceptic MP Sir Bill Cash, who chairs the cross-party Commons committee, said the lack of treaty change would mean there is "no certainty that they will be delivered to the British people".
He added: "Voters in the forthcoming referendum must be aware of this when they make their choice as to whether to vote to remain in the EU or to leave the EU."
Analysis
By Laura Kuenssberg, BBC political editor
The prime minister has already acknowledged, very publicly in a speech last month, that restricting benefits for Europeans working here, whether they be Poles, Italians, Spaniards, Romanians or any others, will be a difficult political sell for other countries, to put it mildly.The wider point about this particular plan is that it challenges a fundamental principle of the EU - that all of its citizens are equal, and entitled in law, to the same treatment and protections.
And sources familiar with what's going on suggest the problem may not just between the UK and many other countries in the EU on this issue, but there is also unease in the government in Westminster.
Read Laura's full analysis
'Ever-closer union'Mr Cameron has set out his demands for EU reform, focusing on four areas:
- Protection of the single market for Britain and other non-euro countries
- Boosting competitiveness by setting a target for the reduction of the "burden" of red tape
- Exempting Britain from "ever-closer union" and bolstering national parliaments
- Restricting EU migrants' access to in-work benefits such as tax credits
It said changes to competitiveness were "already being addressed" and did not think the "ever-closer union" was "legally significant".
Reforms aimed at reducing net migration would not have a "large-scale effect on immigration numbers", the report said.
Mr Cameron's tactics would "not deliver the legally binding and irreversible agreement leading to reform of the EU nor a fundamental change in the UK's relationship with it envisaged by him", it added.
The committee also criticised the "reactive and opaque" handling of the renegotiation and warned "presenting Parliament with a 'fait accompli' could give rise to legitimate concerns about the accountability and transparency of both the process itself, and its outcome".
The four-year waiting time has encountered resistance from other countries and the government has said it would consider alternative proposals that would reduce levels of EU migration.
No deal is expected at this week's summit, which European Council President Donald Tusk says should "pave the way" for an agreement in February.
BBC political correspondent Alex Forsyth says European leaders have warned that the amendment of EU treaties is unlikely to be achieved before the referendum because of the time it would take.
It is understood the government could instead seek a binding agreement with the EU, with treaty change to follow, she added.
A government spokeswoman said: "The prime minister has been clear that the reforms we are seeking will need to be legally binding and irreversible, and that some will require treaty change. His position hasn't changed.
"As the prime minister has said, we must have agreement to such treaty change before the referendum. Ultimately, it is for the British people to decide whether or not the UK remains in the European Union."
Meanwhile, two polls suggest UK voters are almost equally split on whether or not to quit the EU.
An ICM poll for the Vote Leave campaign found that 42% of the 2,053 surveyed would vote to remain a member, 41% would vote to leave, and 17% were undecided.
A poll by Survation for the Alliance for Direct Democracy in Europe, a European political party including UKIP, found that 42% of the more than 10,000 asked wanted to leave the EU, 40% wanted to remain, and 18% were undecided.
