More senior staff are expected to be on hand during the night.
Bankers, traders and analysts are preparing for a night in the office on 23 June to respond to exit polls on the EU referendum that could spark wild swings in foreign exchange and bond markets.
In echoes of the preparations for the Scottish referendum in 2014, major City firms will have teams of experts available through the night to respond to customer queries and demands to move money.
The first exit polls will be published at 10pm on 23 June. Although the definitive result of the vote will not be known until the following day, exit polls could be seized on by foreign exchange dealers and interest rate experts.
The value of sterling has responded to opinion polls during campaigning, falling on Monday as polls showed a vote for Brexit was more likely.
Many banks already have 24-hour trading operations with activity passing from London to New York to hubs in Asia. But more senior staff are expected to be on hand during the night of the referendum result.
JP Morgan, which last week warned it could cut up to 4,000 jobs in the UK in the event of the country leaving the EU, has booked hotel rooms to keep staff near its Canary Wharf head office, Bloomberg reported.
Other banks such as Morgan Stanley, Royal Bank of Scotland and Lloyds Banking Group are among those poised to have teams working through the night.
The Bank of England is also making preparations in the run-up to the vote. Next week it will hold the first of three special funding operations intended to ensure banks are not starved of cash in the days surrounding the vote. The extra funding will be made available to commercial banks on 14, 21 and 28 June.
This article was written by Jill Treanor, for theguardian.com on Monday 6th June 2016 18.18 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010