Britain wants an agreement on European Union treaty change as part of a bid to renegotiate its relationship, but accepts that treaty change may not be fully ratified by the time of the referendum, British government sources said.
"These reforms we are seeking will require treaty change. There will need to be agreement on that treaty change by the time of the referendum," one source said.
"We are not expecting that the changes to the treaty will have been ratified," another source added.
EU officials involved in preparing for the negotiation have long said that a change in treaties ratified by all 28 states cannot be completed before Cameron's end-2017 deadline for a referendum. However, some binding pledges among member governments to make future amendments may well be possible.