UK insurers have scotched claims by pro-Brexit campaigners that leaving the European Union will have no industry repercussions.

The industry, with 34,000 people directly employed in the commercial insurance sector, faces lay-offs and the loss of global trade benefits that come with EU membership, according to a position paper from Lloyd’s of London, the International Underwriting Association of London and Fidelis Insurance.

London risks losing its position as a leading insurance capital should next week’s referendum produce a vote to leave the EU.

“Vote Leave claims the UK will continue to trade freely with the EU after a vote to leave,” the paper says. “The only certainty is that the circumstances in which the London market trades today would change dramatically.

“Britain will have to negotiate, not renegotiate. We will have to start from scratch to get a trade deal with the EU.”

It is impossible to estimate how long negotiations will take or the politics involved, adding to a period of uncertainty and instability for the business community, the paper says.

EU membership provides the London market with unencumbered access to 443 million consumers from the bloc.

“That business will only increase as they thrive unless it is choked off, and diverted to competing insurance hubs and other EU member states in a post-Brexit Europe,” the paper says. “EU membership, without a shadow of a doubt, is the most attractive option for the London market, and we believe for all of Britain.”

About 16% of the London market’s business worth £9.6 billion ($18.7 billion) can be traced to EU members, and the figure is growing annually.

The London market contributed £12 billion ($23.3 billion) to the UK economy in 2013.