The UN Convention intends to improve the rights of people with disabilities
The UK government has been criticised for failing to bring new rights for disabled people into law.
Ministers were supposed to have ratified the UN Convention on the rights of people with disabilities by the end of 2008.
A committee of both Houses of Parliament said this was "extremely disappointing" and risked "alienating" disabled people.
The government says it hopes to complete ratification by the spring.
It also says it has to ensure the convention can be implemented.
The UK was among the first of 137 countries that have signed the convention so far. Less than a third of those have ratified the treaty.
The legislation aims to guarantee equal treatment for disabled people in education, employment and every other area of life.
The delay has come because ministers have been working on a series of reservations - or opt-outs.
The report by the Joint Select Committee on Human Rights strongly criticises the government for a "lack of transparency" and an "unacceptable" failure to adequately consult disabled people over the proposed opt-outs.
These relate to immigration, education and the armed forces.
Committee Chair, Andrew Dismore MP, said: "The UK has led the field in pushing for the acceptance of this convention and advocating the rights of people with disabilities to equal treatment.
"That is why we are particularly disappointed at the trouble we and disability organisations have had getting information about the large number of legal exceptions the government wishes to make to this convention, and at the delays in ratifying it.
"The government should ratify by spring 2009. If reservations are necessary they should be fully justified and compatible with the convention.
"And we call on the government to consult properly with the people who will be affected by these policies as they develop them".
Disability campaigner and founder of the UN Convention Campaign Coalition, Rachel Hurst criticised ministers for not revealing what their reservations are based on.
RIGHTS GUARANTEED IN TREATY
To make your own decision
To say no to being placed in an institution
To say no to medical or psychological treatment
To live in the community
Equal opportunities for all
The removal of barriers to participation in daily life
She said: "The government has not been really open with us about exactly what the reservations are.
"One is around immigration and we don't understand what that's going to be."
She added: "The armed forces are being rather ridiculous about trying to say that they can't possibly employ disabled people in the field but of course nobody's expecting them to and the convention doesn't require it."
Dr Alice Maynard, who chairs disability rights charity Scope, said the government had dragged its feet on this "hugely important" treaty.
She said: "We hope the select committee's findings will encourage the government to accelerate its efforts to ratify this vital treaty in full and without further delay to ensure disabled people in the UK have full rights."
A Department for Work and Pensions spokesperson said: "The UK takes the ratification of international treaties seriously and we do not ratify them until we are in a position to ensure we can implement their provisions.
"The work involved has necessarily been complex and time-consuming."