Blackrock president Rob Kapito goes on to say that international companies with offices in the UK are already looking at possibly moving staff abroad.
In an interview with Today from his New York office, Morgan Stanley president Colm Kelleher also talks of the lack of clarity about what exactly Brexit will look like.
"It is that uncertainty which is causing problems," he says.
"You will see a slowdown in investment into the UK because people and corporations like certainty before they invest. So the sooner we get some clarity on where we're heading the better."
Blackrock president Rob Kapito goes on to say that international companies with offices in the UK are already looking at possibly moving staff abroad.
"I don't think there's any firm, any good firm, that has not already started looking at real estate in different areas outside the UK in case they have to move larger operations," he says.
"There's no-one that doesn't have people looking at tax implications, investment implications, manufacturing implications, so being one of the largest investors in the globe, we're very concerned about the future economics to a lot of corporations and how it'll affect them."
Colm Kelleher, president of US bank Morgan Stanley, tells the Today Programme that London's financial services will be hit by the UK leaving the European Union.
"I do believe, and I said prior to the referendum, that the City of London will suffer as a result of Brexit. The issue is how much," he tells Dominic O'Connell.
New figures showing that nearly 5,500 UK finance firms use "passporting rights" to trade in the EU has focused minds again on the cost of Britain leaving the single market.
New figures showing that nearly 5,500 UK finance firms use "passporting rights" to trade in the EU has focused minds again on the cost of Britain leaving the single market.
"I am convinced that London will retain its reputation and prestige as a global financial services centre, but clearly some size of our businesses will have to be moved out of London and into Europe with the absence of any passporting agreement," Mr Kelleher says.
"It's very hard to ascertain what that means at the moment. I do think generally though that capital markets in Europe will shrink as a result of this."
So not everyone is in agreement and only time will tell who was right.