What happens to the City after Brexit?
One of the major uncertainties facing financial services will be the level of access to EU markets they are granted in the event of a Leave vote.
As it stands, the City operates under uniform EU rules, which allows Britain to export financial services worth more than £20bn, or 1.1pc of GDP, to the EU.
Should Britain enter some form of associate membership after a Brexit, it would throw into doubt this "passporting" of financial services.
UK firms and regulators would have to ensure they still comply with the raft of Brussels' existing regulations in order to regain access to the single market.
For some bodies, such as lightly regulated hedge funds, this would hardly be a hindrance to their operations. However, others such as clearing houses, could face serious disruption to their business.
The ECB has already tried to pass laws forcing all clearing houses to be based on the continent. Britain successfully overturned the ruling by taking the case to the European Court of Justice in a landmark ruling last year.
"Euro-area institutions may use any British exit to try and bring a greater share of euro-denominated trading and settlement within the eurozone, taking business away from the City," says Simon Wells at HSBC. "This is a very real threat."
A messy Brexit - in which relations between the UK and the rest turn sour - could represent a "worst-case scenario", where the UK's financial services are no longer deemed equivalent to European rivals under the EU's regulatory regime, says Edward Chan at law firm Linklaters.
UK bank might have to then set up a branch in every single one of the European jurisdictions and have each of those authorised
Edward Chan, Linklaters
"For a US bank setting up a subsidiary in the UK, it would be difficult to see how they could then access the European markets in the way that they do now," says Mr Chan.
"For UK banks, they might have to then set up a branch in every single one of the European jurisdictions and have each of those authorised. Each of them will come with a local management requirement. It would be a real lack of efficiency for any UK entity to have to do that."