The Financial Conduct Authority has plans in place for a financial war room to take action in the event of a no-deal Brexit.
The war room will keep a close eye on the IT changes that will be required over the weekend following 29 March.
It is understood that the regulator will keep in close contact with leading City firms to watch for potential disruption in the financial markets.
The FCA will also liaise closely with Whitehall and Threadneedle Street.
An FCA spokesman said: “As part of our planning for all scenarios we have put in place contingency measures in the event of a hard Brexit.
“We will have teams in place throughout the weekend of exit to monitor the situation and respond as needed, working closely with the Treasury and the Bank of England.”
The Financial Services Register of authorised firms will need to be updated constantly as EU providers sign up to deal with UK clients.
Hundreds have already applied, knowing that their “passporting right” to operate here as EU members will expire.
Previously, the FCA warned drivers heading across the Channel to make sure they have Green Cards to prove that they are insured.
Holidaymakers are advised to check their travel insurance, to make sure they are covered for problems with flights.
Insurers have been told to treat customers fairly if they make claims for travel disruption and “not to reject claims unreasonably”.
Travellers will also need to check that they have the health cover they need if the EHIC card which entitles UK tourists to treatment across Europe ceases to function.