The European Central Bank would publicly pledge to backstop financial markets in tandem with the Bank of England should Britain vote to leave the European Union, officials with knowledge of the matter told Reuters.
The preparations illustrate the heightened state of alert ahead of the June 23 referendum.
The referendum will help determine Britain's future in trade and world affairs and also shape the EU.
The pound and euro have lost value on fears a Brexit could tip the 28-member bloc into recession.
Such an announcement from the ECB would come on June 24 if an early-morning result showed that British voters had chosen to leave the EU, according to the sources.
The aim is to underpin investor confidence across Europe and contain further market jitters.
"There will be a statement to do whatever it takes to maintain adequate market liquidity," one senior central bank official said, who spoke on condition of anonymity.
The ECB's pledge would involve opening so-called swap lines with the Bank of England, allowing euros and sterling to be exchanged and effectively making unlimited funding in both currencies available to European banks, the sources said.
The ECB and Bank of England declined to comment.
The Bank of England said last month that possible "heightened uncertainty" due to the vote may make it harder for banks to tap their usual sources of foreign currency and that it would keep its operations, including swap lines, under review.
The ECB used similar arrangements with the US Federal Reserve to unlock extra dollars during the financial crisis and after the 9/11 attacks in the US.
Providing extra funds to banks after a Brexit vote would ease pressure on them and reduce the potential for panic as financial markets digest the result on Friday June 24, shortly before closing for the weekend.
However, the ECB is limited in what it can do, beyond releasing funds for banks.
Extending its money-printing programme, which has already saturated markets with more than €1 trillion and is gradually running out of bonds and securities to buy, would likely be contested by some countries such as Germany.
Such a move could only be considered if the market fallout from a Brexit were to weigh on the regional economy over the longer term.
In the days around the referendum - which is expected to be closely contested - governors of European central banks will hold a series of meetings, some of which were scheduled independently of the vote.
These gatherings may present an opportunity to fine-tune their response.
Bank of England Governor Mark Carney is due to meet ECB President Mario Draghi and European peers in Frankfurt on June 23, the day Britons cast their vote.
Governors from across the 19 European countries that use the euro have also scheduled a telephone conference call the following day to decide their response to the vote, two people with knowledge of the matter said. A statement could follow.
On the weekend after the vote, central bank chiefs from around the globe will gather in Switzerland for the annual meeting of their umbrella body, the Bank for International Settlements.
The Bank of England has already sought to avert any liquidity squeeze ahead of the vote by providing injections of cheap funding for banks ahead of the vote.
The first of two such batches was released today, prompting banks to request £2.5 billion for six months.
Meanwhile, Canada's Finance Minister Bill Morneau said today that its trade agreement with the European Union would be stronger if Britain remained part of the bloc.
He said that Britain would be stronger if it remained a member of the union and that Canadian businesses with investments in Britain felt at risk with the possibility of a Brexit.
