Brexit "war bunkers" are being opened, extra staff brought in and sleep-deprived workers plied with pizzas, sweets and coffee to keep them going throughout the night as the City beds in for expected high levels of pre- and post-referendum trading.
Barclays, HSBC, Lloyds, Societe Generale, Deutsche Bank, Royal Bank of Scotland and JP Morgan will all have heightened staffing levels, especially in foreign exchange.
“In the FX market ,you could in theory have Asia cover a lot of the overnight stuff. But clients like to speak to sales and trading people that they actually know. And with traders in particular, the person that actually has experiences of trading the nuances of sterling even in a volatile market," one City bank source told City A.M.
JP Morgan have booked out hotel rooms in Canary Wharf to keep staff nearby.
Trading is expected to start early.
City Index, the spreadbetting and trading company, will have around 150 staff in its Finsbury Circus office at 2am – around 10 times the normal Thursday night staffing – ahead of an expected rush of trading between 3am and 5am.
Michael Hewson, chief market analyst at CMC Markets UK, was among those preparing himself for an early start.
"I think most people in the City are doing [the same]. I will be walking out of here as the polls close, going to my hotel room and coming back about half-past three, four o'clock in the morning," he said.
CMC Markets is working a shift system, with others working during Hewson's down time.
While some law firms have set up EU referendum hotlines, Clifford Chance went a step further, setting up Brexit "war rooms".
The Magic Circle firm opened up its 24-hour rapid response unit on Thursday and will remain active for as long as necessary in the event of a Brexit.
"We have been working on the likely implications of a Leave vote for several years now and have established a dedicated 24-hour rapid response unit," senior partner Malcolm Sweeting said.
"Led from London, our EU Referendum Operations Room will connect a core team to specialist advice in the firm’s offices in major financial centres across Europe, Asia Pacific and the Americas.
"The facility will be staffed from the opening of the polls for as long as necessary to ensure we can quickly and effectively respond to the needs of our clients, wherever they are based."
Chris Lodge, City Index’s global head of client management, told City A.M.: "I think there's a real sense of excitement. We’ve got some pizza coming up and the coffee’s pretty good in this office, so we should survive. I hope."