LONDON (Reuters) - The theft of money from accounts at Tesco bank is unprecedented in Britain and how it happened is still unknown, Britain's financial watchdog said on Tuesday.

Tesco Bank halted online transactions from all current accounts on Monday after money was stolen from 20,000 accounts in Britain's first such cyber heist.

"When I heard about this on Sunday I thought this looked unprecedented in the UK," Financial Conduct Authority Chief Executive Andrew Bailey told a parliamentary committee.

"It's too early to give a comprehensive account of what the root causes are," Bailey told the Treasury Select Committee.

The incident appeared to take place on the debit side of Tesco Bank's online banking, he said.

"It requires further urgent analysis to understand the broader implications of this," Bailey said.

The bank has said it will reimburse customers by the end of the day on Tuesday, he said.

(Reporting by Huw Jones; editing by Jason Neely)