Facebook has suspended a Canadian data firm that played a key role in the campaign for the UK to leave the EU.
The social media giant said AggregateIQ (AIQ) may have improperly received users' data.
It cites reported links with the parent company of Cambridge Analytica (CA), the consultancy accused of improperly accessing the data of millions.
AIQ denies ever being part of CA, its parent company SCL or accessing improperly obtained Facebook data.
The Vote Leave campaign paid AIQ £2.7m ($3.8m) ahead of the 2016 EU referendum.
An ex-volunteer with the campaign has also claimed Vote Leave donated £625,000 to another group to get around campaign spending limits, with most of the money going to AIQ. Vote Leave has denied any wrongdoing.
AIQ's website once quoted Vote Leave chief Dominic Cummings as saying: "Without a doubt, the Vote Leave campaign owes a great deal of its success to the work of AggregateIQ. We couldn't have done it without them." The quote has since been removed.
In total, AIQ was given £3.5m by groups campaigning for Brexit, including Vote Leave, the Democratic Unionist Party and Veterans for Britain. The UK's Electoral Commission reopened an investigation into Vote Leave's campaign spending in November.