France's wealthiest woman, L'Oreal heiress Liliane Bettencourt, has promised to declare all of her foreign assets to comply with French laws after she was caught on tape allegedly plotting tax evasion, according to media reports.

"I have decided to declare all of my family assets that are currently abroad in co-operation with the French tax authorities," the shampoo and cosmetics billionaire said in a statement.

Secret tapes revealed that Bettencourt, 87, had allegedly conspired to hide money in Swiss bank accounts while making donations to friends in the governing UMP party. She has a fortune estimated at $20 billion.

Bettencourt said funds from an old Swiss bank account and an insurance policy purchased for one of her grandsons would be declared to tax authorities among other sources of revenue.

Reports said that transcripts of tapes published on the Mediapart website suggested Bettencourt funnelled €80 million (around $98.5 million) into Swiss bank accounts and planned to move the funds to Singapore after France signed a tax cooperation deal with Switzerland.

The French government, along with a number of other nations such as the US, Germany and the UK, is seeking to clamp down on people using offshore bank accounts to allegedly evade tax. In some cases, countries have gone to the lengths of acquiring data stolen by former private bank staff.

The French authorities have the details of clients of HSBC's Swiss private bank after purchasing a stolen disk from Herve Falciani, a former HSBC computer specialist. Additionally, German states have acquired data stolen from Credit Suisse.