Financial firms could see the cost of bringing skilled European workers to the U.K. increase by up to 300% after Brexit if Britain does not reform its immigration system, according to the nonprofit organization TheCityUK.

Research by TheCityUK, which promotes the U.K. financial sector, and Ernst & Young warned that, if Britain does not strike a deal with the EU on the movement of people, employees from Europe working in London will need to use the "Tier 2" work visa system.

This will lead to an increase in visa applications and could lead to far higher costs for banks, insurers and other financial sector companies.

The report made various recommendations, including the introduction of a short-term immigration category to allow some people to work in the U.K. for six months without needing to apply for a visa.