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The sister of champion swimmer Yusra Mardini, reached the Greek island of Lesbos in 2015 after they swam for their lives when the inflatable boat they had left Turkey in started taking on water.

The four face charity workers were charges including belonging to a criminal organisation, money laundering, espionage and breaches of immigration laws, which they strongly denied.

Although resident in Germany, Mardini returned to Lesbos - a key point of entry for migrants trying to reach Europe - to help other refugees.

She worked with the Emergency Response Center International charity but was arrested in August on severe criminal charges, prompting international concern.

Mardini's sister Yusra was a member of the refugee team at the 2016 Rio de Janeiro Summer Olympics.

Haris Petsikas, the charity members' lawyer, said all four were released from prison pending their trial after paying bail.

He said Mardini and Sean Binder of Germany were now free to leave Greece but have no immediate plans to do so. The other two suspects, both Greeks, also paid bail but were forbidden to leave the country.

Petsikas said the trial was not expected to start for at least six to eight months.

Greek police claimed the four allegedly facilitated the illegal entry of migrants to Greece for profit, to raise donations or subsidies for the charity, new in advance of smuggling boats' routes and numbers of passengers and illegally monitored Greek and European coast guard radio traffic.

Most migrants who reach Lesbos make a dangerous journey by sea after paying a high price to smuggling gangs.

Petsikas said the four being released from pre-trial detention was a good sign.

"The evidence that was provided (to authorities) negates the charges," he said.