The Federal Court has given the go-ahead for Switzerland to assist the Nigerian authorities probing the financial dealings of the late dictator Sani Abacha.
Judges rejected an appeal aimed at stopping the transfer of financial documents relating to Abacha's bank accounts in Switzerland.
However, the court said files would not be handed over until Lagos guaranteed defendants would get a fair hearing.
The former Nigerian leader, who died of an apparent heart attack in 1998, is believed to have stashed billions of dollars abroad, including Switzerland.
Funds worth more than $600 million (SFr813 million) are currently blocked in Swiss bank accounts.
"The Swiss authorities are ready to collaborate with Nigeria in order to return this money," said Folco Galli, a spokesman for the justice ministry.
A group made up of Abacha's family and associates - including several companies based in the Virgin Islands, Liechtenstein and Panama - appealed last year to Switzerland's highest court to block legal assistance agreed in January 2001.
They claimed an investigation into the former dictator's dealings had already been carried out in Nigeria and that any suspects had either been given an amnesty or had settled the matter with the authorities.
The court rejected the appeal, however, saying that the investigation they referred to was not an official one and its findings could therefore not be used as a basis for an appeal.
It also said the appellants' case was invalid, as it referred to accounts opened under false names.
Around $600 million was frozen by the Swiss authorities in 1999 after Nigeria asked for assistance in investigating the financial network allegedly set up by Abacha.
The extent of Switzerland's involvement with Abacha and his family was revealed by an investigation carried out by the Federal Banking Commission.
The commission discovered that six Swiss banks violated their obligations in their dealings with the ex-dictator.
In particular, Credit Suisse, the country's second largest financial group, came under fire, after being found to have accepted SFr317 million from one of Abacha's sons.
The findings caused embarrassment to Switzerland, because much of the money involved found its way into bank accounts after a law intended to clamp down on money laundering was introduced.
swissinfo with agencies
The Federal Court has paved the way for Switzerland to cooperate with the Nigerian authorities in their probe into ex-dictator, Sani Abacha.
The court threw out an appeal by Abacha's aides seeking to block Switzerland's involvement.
Abacha is suspected of stashing billions of dollars in overseas accounts.
More than $600 million has been frozen in Switzerland.