Prosecutors in Geneva have fined a Swiss bank for failing to alert money laundering authorities about more than 100 million US dollars (£75 million) from Saudi Arabia that went to former Spanish king Juan Carlos and his ex-lover, but dropped possible charges against his associates in the case.

The Geneva prosecutor’s office partially dropped an investigation opened three years ago into five people for alleged money laundering, while deciding that Mirabaud bank had failed to properly communicate with the Swiss money laundering office.

The Swiss investigation was prompted after reports in Spanish media about possible illegal payouts benefiting the king in the awarding of public contracts to Spanish companies — funds that could have been hidden in Swiss banks.

Some of these payouts were suspected to be linked to commissions on a high-speed rail project from Medina to Mecca in Saudi Arabia.

Juan Carlos himself was not among the five being investigated in this particular investigation.

Investigators did find that Juan Carlos received on August 1 2008 — before he stepped down as king — through a foundation known as Lucum, the sum of 100 million US dollars from the Saudi finance ministry, the prosecutor’s office said.

It also turned up millions more received by the king or his former lover Corinna Larsen, a Danish/German businesswoman.

The prosecutor’s office said use of a foundation to take in the funds “demonstrated a desire” to hide funds, but “the investigation could nevertheless not establish sufficiently any link between the amount received from Saudi Arabia and the conclusion of contracts for the construction of the high-speed train”.

It ordered Mirabaud to pay a fine plus legal fees totalling 200,000 Swiss francs (£163,000) for its failure to alert the money laundering office about Ms Larsen’s personal account.