Achilles has already fulfilled a similar role during the 2017 FIFA Confederations Cup, also held in Russia.
"There will be a special press conference and it [Achilles] will get a fan ID. The decision has already been made, the papers have been signed," Haltunen said.
READ MORE: Hermitage Museum Cat Achilles May Become Oracle for 2018 FIFA World Cup
The spokeswoman said that Achilles had recently had holidays living with a family where he put on about a kilogram (over 2 pounds) and weighs now 4.72 kilograms.
Preparing for the World Cup, Achilles will visit several children's hospices and maybe senior houses.
The cat was chosen as an oracle for the Confederations Cup because of his good resistance to stress and well-developed people skills. Achilles, who was born with a hearing impairment, was also chosen in part due to his analytic abilities and unusual behavior.
The cat made his predictions of a potential winner by choosing one of two plates of food, each marked with the flag of one of the teams.
Cats have been living in the museum since the early 18th century, when Peter the Great brought a cat from the Netherlands to his residence. Catherine the Great officially conferred the status of painting gallery guardians on the cats living in the palace. The tradition of keeping cats to protect the galleries from rodents was suspended only during the blockade of the city in the World War II.
Russia will host its first FIFA World Cup from June 14 to July 15. The matches will take place in Moscow, St. Petersburg, Kaliningrad, Volgograd, Kazan, Nizhny Novgorod, Samara, Saransk, Rostov-on-Don, Yekaterinburg, and Sochi. The first game of the World Cup tournament — Russia vs. Saudi Arabia — will be held in Moscow on June 14.
The most famous oracle animal was Paul the Octopus, who forecast results of matches of the UEFA Euro 2008 and FIFA World Cup 2010. Paul made only two mistake in 14 matches that were subjected to his forecasts.