The spokesman said the agreement means Samantha Orobator, who is pregnant, and John Watson could be returned to Britain within weeks, CNN said.
The official said Orobator and Watson must apply for return under the agreement, which British and Laotian officials brokered in May. British Foreign Office Minister Chris Bryant signed a memorandum of understanding with Laos that speeds up the prisoners' return.
Orobator is in her third trimester and can travel safely for two more weeks, Bryant said. She was jailed in August after being arrested with heroin at the airport in Vientiane, officials have said. After a trial last month, she was sentenced to life in prison for drug trafficking. The Foreign Office said Watson also was jailed on drug charges.
Mystery surrounds the circumstances under which Orobator became pregnant while in prison, CNN said. She has told her mother she wasn't raped and that the father wasn't a prison guard. A state-run Laotian newspaper reported Orobator said she impregnated herself with sperm from another prisoner.