By Geoff Adams-Spink
Age & disability correspondent, BBC News website
The Convention has been ratified by more than 40 countries
The UK's equality body - the Equality and Human Rights Commission (EHRC) - has criticised the government's delay in signing the UN disability treaty.
And the EHRC has written to the government asking it to explain the large number of proposed opt-outs.
The UN Convention on the Rights of Persons with Disabilities came into force in May last year.
The government says it remains "fully committed to ensuring equality for disabled people".
The UN convention sets out the rights of an estimated 650m disabled people and the responsibilities of governments in removing barriers to inclusion and promoting equality.
So far, it has been signed by 137 countries and ratified by 44 of them.
Ratification means that a country accepts its legal obligations under the convention and enacts any necessary legislation.
The EHRC says that the government has passed its self-imposed deadline for ratification by the end of 2008.
And it says the UK has requested more "reservations" or opt-outs than the rest of the countries that have already ratified the treaty put together.
The Commission has written to four government ministries to express its concern that the UK's "well-deserved reputation as a leader in promoting the rights of disabled people" risks being undermined.
Soon as possible
The disability minister, Jonathan Shaw, said the UK government was one of the first to sign the convention.
"The UK approach to ratification is not to ratify any international treaty until it is in a position to implement the provisions and comply with them," he said.
"We agree with the Commission that the UK should ratify as soon as possible, and we are working hard to achieve this."
The government is also being criticised for a lack of transparency and a reluctance to consult disabled people and disability organisations.
The EHRC has written to the secretaries of state for defence, education and work and pensions and the home secretary, asking for details of all proposed reservations to be placed in the public domain.
It also wants each of the departments concerned to publish their justifications for requesting opt-outs.
Writing on behalf of the EHRC, commissioner Baroness Jane Campbell said:
"I know that you will understand why any perceived resistance to openness and consultation on a matter of ratifying the convention would risk damaging the government's reputation in the area of disability rights, where it should otherwise be extremely proud of its achievements."
The shadow disability minister, Mark Harper, has described the delay in ratifying the first human rights treaty of the 21st century as "staggering".
"This dithering is unacceptable," he said.
"The government needs to be honest where reservations are required but then to get a move on and ratify the convention with the necessary reservations in place."