The UK Gambling Commission has "temporarily" suspended the licence of betting exchange Matchbook but the reason for the move is unclear.
Matchbook contacted customers on Monday to say they would be closed for all betting and casino activity from Tuesday and promised they would be "back soon". They were also told they would be able to withdraw funds and that open positions would be settled.
In an email to customers, Matchbook said: "Today the United Kingdom Gambling Commission has taken the decision to temporarily suspend the operating licence of Triplebet Limited (t/a Matchbook).
"As a result, from 23.59 on 17 February 2020 we will be briefly closed for all betting and casino activity in the United Kingdom. During this time we will continue to settle all open positions and you will have access to your account to withdraw your funds.
"Matchbook has been in regular contact with the Gambling Commission and has agreed a path forward where it will deliver on a number of conditions. We will be back soon and we are committed to providing a betting exchange which adheres to very high standards."
The Racing Post has contacted the Gambling Commission for comment.
Matchbook have been in operation since 2004 but were acquired in 2011 by a group of investors with backgrounds in British sports betting and banking.
Horseracing has increasingly become a focus for Matchbook, which was among the firms to sign up to the Authorised Betting Partner scheme in 2016.
They have sponsored a number of races in Britain and Ireland, including at Cheltenham, Goodwood and Ascot, where the Grade 1 Matchbook Clarence House Chase is the group’s headline race.