A forensic scientist looking at the cases of the Disappeared will also be able to work on the Lisa Dorrian case.
Lisa Dorrian's body has never been found
Lisa, 25, was last seen at a party on a County Down caravan site in February. Her body has never been found.
Northern Ireland Secretary Peter Hain said the police would be able to use the expert to undertake other work on an independent consultancy basis.
The move was confirmed in a letter to North Down UUP MP Lady Sylvia Hermon, who said "common sense had prevailed".
"No longer is there any objection to the involvement of the forensic expert in the search for Lisa Dorrian," she said.
"Since engaging this particular expert as an independent consultant, the police have assured the Dorrian family and me that he will continue to take an interest in Lisa's case."
Lady Hermon said it would "give hope to Lisa's family that the day and hour of recovery of her remains are brought that bit closer".
Police believe members of the Loyalist Volunteer Force may have been involved in her killing. The splinter paramilitary group announced on Sunday it was standing down.
The forensic scientist is being funded by the British and Irish governments to help locate the remains of five people abducted, murdered and secretly buried by the IRA during the Troubles.