Citi has joined a long line of independent financial groups that have warned the UK's economy would be damaged if Britain were to leave the European Union.
According to a media interview, Jim Cowles, Citi chief executive officer for Europe, Middle East and Africa, said the investment banking giant's clients are becoming increasingly concerned that the UK will no longer be an attractive and viable hub for services should Britain leave the EU- dubbed the 'Brexit'.
"It is not that international companies will stop investing in Britain, but their investment just will not be at the scale we have become accustomed to," said Cowles.
On 21 January, the International Monetary Fund said Britain's economic recovery will be even stronger than thought during 2014.
The IMF said the UK economy would grow by 2.4% across the year, up from its previous estimate of 1.9% and the biggest increase of any country in the world. Growth will then slow slightly to 2.2% in 2015.
Prime Minister David Cameron promised voters he would renegotiate the terms of Britain's EU membership before holding an in-out referendum by 2017 if his ruling Conservatives were returned to power after elections due in May 2015.
UK Chancellor George Osborne has already warned the EU directly that its faces losing the UK and other countries' membership if it does not reform.
The Citi chief's warning follows a number of independent groups that have voiced their concerns as well as a number of corporation giants that have revealed that their investment in the UK may be pared back in the event of a 'Brexit'.
Consumer goods giant Unilever has warned Britain that that it will rethink bringing its billions of euros worth of investment into the country is the UK ends up leaving the European Union.
"We are very concerned about the overall competitiveness of Europe vis-a-vis the rest of the world," said Paul Polman, chief executive at the Anglo-Dutch group, in a media statement.
Meanwhile, one of Britain's biggest employers, Airbus also said that the benefits of an alternative economic model needed to be proven as concerns are mounting over the UK leaving the EU.
Elsewhere, independent think tanks have highlighted research that show that Britain that it would struggle to maintain trade links with European Union members and would give up 30% trade growth if it left the 28-nation bloc.
According to The London-based Centre for European Reform (CER), Britain's trade in goods with other EU member-states was 55% higher than would be expected given their output, proximity and other traits. This translates to a 30% boost in goods trade under EU membership.
On top of that, the EU-US agreement is expected to boost the bloc's economy by €120bn ($161bn, £100bn) and the US economy by €90bn, according to independent research.
Furthermore, the global economy would get a €100bn boost from the deal.
As of 2011, the US and the EU maintain a total of nearly $3.7tn in investment in each other's economies.