The UK insurance industry will lose more than it will gain if Britain votes in a referendum next month to leave the European Union (EU), according to a report based on interviews with senior executives.

All but one of the 20 executives – drawn from 13 insurers, reinsurers and brokerages – interviewed in February and March by global law firm Kennedys are in favour of remaining in the EU.

The biggest concern in the event of a “Brexit” is the industry-wide uncertainty that will follow, and the difficulty of accurately assessing the potential impact as there are far too many unknowns.

“Almost all respondents felt that the costs of an EU exit would outweigh any benefits,” the report says. “As a result, there was near-universal support for the UK to remain within the EU.”

The consensus view is it will take at least five years for the UK to negotiate its exit, exacerbating more uncertainty for the business community.

A “leave” vote could also trigger an exodus by firms relocating to other financial hubs, jeopardising London’s position as a major insurance hub.

“If firms choose to locate to a jurisdiction where the relationship with the EU will be more predictable, we may see business start to move within a relatively short period,” the report says.

“These concerns are strongly felt in the London Market where the client base is global and the workforce is highly mobile.

“The combination of globalisation and digitisation is weakening the need to maintain a physical presence in London.”

The UK insurance industry manages investments of £1.9 trillion ($3.9 trillion), employs 334,000 people and enjoys a £2.1 billion ($4.3 billion) trade surplus with the EU in terms of insurance and long-term savings products.

UK insurers contributed £29 billion ($59.2 billion) to the British economy in 2012.