Stock trading app Robinhood is being investigated by the Securities and Exchange Commission for failing to disclose that it sold clients' orders to high speed trading firms, according to the Wall Street Journal.

Robinhood did not disclose, until 2018, that it received payments from high-speed trading firms for sending them client orders to buy or sell stocks and options, says the WSJ.

The fintech giant could face a fine of more than $10 million if it agrees to settle the investigation, which is at an advanced stage, says the Journal, citing sources. However, a fine has not yet been negotiated.

While payment for order flow is common for retail brokerages, critics say it creates a conflict of interest. Robinhood only began disclosing the practice on its site in 2018.

A darling of the fintech scene, Robinhood hit an $11.2 billion valuation last month on its latest funding round.

The firm has added millions of funded accounts just this year and in recent weeks it has seen record revenue growth, with investors keen to take advantage of market volatility during the Covid-19 pandemic.