The two brothers, of Iranian origin, were identified on the charge sheet as Payam Kia, 35, and Peyman Kia, 42.
They are accused of spying for Russia's GRU military intelligence service from 2011 to 2021.
Peyman Kia served in Sweden's intelligence service, Sapo, and intelligence units in the Swedish army.
“It has been a complex investigation concerning a crime that is very difficult to investigate and the suspicion concerns very serious criminality directed against Sweden’s intelligence and security system,” National Security Unit chief prosecutor Per Lindqvist said.
“The information that has been obtained, transmitted and divulged could, by the fact that if it comes into the hands of a foreign power, result in detriment to Sweden’s security,” he said in a statement.
He later told a news agency the case involved “extremely sensitive topics,” but declined to elaborate.
According to the charge sheet obtained by the Associated Press, the men “jointly” passed information to the Russian military intelligence service GRU during the period September 28, 2011 to September 20, 2021.
Swedish newspaper Dagens Nyheter reported that Peyman Kia at one point worked for the Office for Special Information Gathering (KSI), the most secret section of the military secret service.
He is accused of illegally acquiring information during his employment with Sapo and the armed forces.
His brother, Payam Kia, allegedly helped with the contacts with “Russia and the GRU, including matters of surrender of information and receipt of compensation.”
According to the charges, Payam Kia “dismantled and broke a hard drive which was later found in a bin” when his brother was arrested. He is accused of “participating in the planning of the deed and handling contacts with Russia and the GRU, including the handover of information and receiving compensation”.
The pair, who face life sentences if found guilty, have denied the allegations, according to Swedish media. A life sentence in Sweden generally means a minimum of 20-25 years in prison.
Defence Minister Pal Jonsson told parliament on Friday the case was “very sensitive” and the country had strengthened its security since the pair's arrest a year ago.
Sapo said it became suspicious of its former employee and a preliminary investigation was launched in 2017.
Mr Lindqvist said the inquiry was started “because there was a suspicion that there was a mole, an insider” within Sweden’s intelligence community.
The domestic spy agency confirmed that Peyman Kia had worked there between 2014 and 2015, and that before that had worked in the Swedish armed forces. “It is during these employments that the suspected acquisition must have taken place.”
Swedish media said that he worked for the armed forces’ foreign defence intelligence agency Must and reportedly worked with a top secret unit under Must which was dealing with Swedish spies abroad. He later worked for Swedish Customs.
“The suspected crime is a risk that every security service is well aware of, although we do everything to counter it,” said Anders Kassman at Sapo.
Peder Ohlsson, head of the armed forces’ press department, said the crimes were “very serious.”
Swedish Defense Minister Pål Jonson said he had been informed of the case, but declined to comment.
The brothers became Swedish citizens in 1994, and the Expressen daily quoted one of the men as saying that he speaks Persian fluently. The younger brother had worked for Sapo, Swedish media reported.
They were arrested in September and November 2021.
Last month, Norwegian police arrested a suspected Russian spy in an Arctic town.
The man, who worked as a scientist at the University of Tromso, was posing as a Brazilian citizen but police believe his real identity to be Russian, public broadcaster NRK reported.
He had been in Norway since 2021 and was believed by the PST, the Norwegian Police Security Service, to have sought to establish himself in the country under a false identity while secretly working for Russian intelligence.
Norway, which shares a border with Russia and is a member of Nato, has arrested several Russians suspected of illegally flying drones or taking pictures in restricted areas in separate incidents recently.
European nations have heightened security around key energy, internet and power infrastructure following September’s underwater explosions that ruptured two natural gas pipelines in the Baltic Sea that were built to deliver Russian gas to Germany.
The damaged Nord Stream pipelines off Sweden and Denmark discharged huge amounts of methane, a potent greenhouse gas, into the air.