Voters in the EU referendum should be made aware there is "no certainty" of any agreed reforms actually being implemented, a committee of MPs said.
The European Scrutiny Committee said David Cameron's proposals "will not deliver the legally binding and irreversible agreement leading to reform of the EU nor a fundamental change in the UK's relationship's" he said he wanted.
And some elements would require treaty change that could not be put in place or guaranteed for the future by the December 2017 deadline set by the Prime Minister for the UK's in/out vote.
In a critical report, the committee 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 move to find ways to prevent eurozone countries outvoting those outside the single currency was "so important that it absolutely requires the security of Treaty amendment to address it", the report found.
And the potential "deep involvement" of the European Court of Justice over measures to curb migration suggested treaty change "will be necessary in order to make the outcome of the renegotiation 'judge-proof'".
It concluded: "Voters are entitled to know the extent to which subsequent Treaty amendment will be required to deliver any new agreement, and how robust and meaningful any guarantees or promises in this respect may be."
Sir Bill Cash, the veteran Conservative eurosceptic who chairs the committee, said: "Our report is clear that the only way in which the Prime Minister's negotiations could be given the legally binding and irreversible effect which he himself has called for - even for minor reforms to the EU, let alone a fundamental change in the UK's relationship with the EU - would be through Treaty amendment, or the equivalent agreement of a Protocol.
"These would be lengthy processes, and each Member State would have to agree using its own constitutional procedures including, in some countries, referendums.
"Whatever the promises made in the negotiations, there is no certainty that they will be delivered to the British people. 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."
Mr Cameron's official spokeswoman said: "The Prime Minister has made clear that he wants to see legally binding, irreversible changes and he has been clear that in some of these areas it is going to need treaty change."