Lloyd's of London, AIG, Allianz and other insurers are ignoring assurances and establishing new hubs in Britain and the European Union before Brexit in March 2019 to ensure access to customers.
The moves come despite a "standstill" transition agreement struck between Britain and the European Union in March of this year which is meant to avoid any such hasty relocations.
"We have urged firms not to wait for and rely on a political process to deliver the answers. This is particularly true of relocation plans, which take two years or more to complete," Hugh Savill, of the Association of British Insurers, said.
Insurers are being driven by the fact that after Brexit, European firms selling policies in Britain, as well as British and other non-European Union insurers with UK bases selling into Europe, will need to have local regulated entities.
Many contacted by Reuters have said they are starting to implement the second phase of their Brexit plans - submitting licence applications, hiring staff and shifting policies.
"We are prepared for a hard Brexit," said Joachim Wenning, chief executive of Germany's Munich Re, the world's biggest reinsurer, which has applied for UK licences.
Such planning has been encouraged by EU regulators who say transition will not be ratified until October and could be derailed without agreement on other parts of Britain's divorce.
"I don't think there is any going back," Paul Merrey, a partner specialising in insurance at consultants KPMG, said.
American insurer AIG said it will open new subsidiaries in Britain and Luxembourg by December, and has begun moving policies from one jurisdiction to another.
Meanwhile, Japanese insurer Sompo's international unit last week received approval for its Luxembourg subsidiary, which it said would start operating before the end of the year.
Even Lloyd's of London, the world's largest commercial insurance market which is synonymous with the capital's financial centre, will have its new Brussels subsidiary ready by January for the policy renewal season kick-off.
"Companies must take their futures into their own hands, and Lloyd's is no different," its Chief Executive Inga Beale said, while Lloyd's market operator CNA Hardy's CEO Dave Brosnan said it is obtaining a licence for a Luxembourg subsidiary.
The Bank of England says that transition, which is meant to give financial firms breathing space until the end of 2020, can be relied on immediately so that EU insurers do not have to seek reauthorisation for UK operations by next March.
It has also eased the rules for deciding if operations of insurers from outside Britain can continue as branches or must convert into subsidiaries with their own capital, an expensive process which only a small number face.
The BoE says UK and EU legislation is needed to ensure 10 million British policyholders with 27 billion pounds in liabilities and 38 million European Economic Area policyholders with 55 billion pounds of liabilities are not hit.
Swiss insurer Zurich is talking to UK regulators about the licence for its general insurance business, which has its EU headquarters in Dublin. Its UK CEO Tulsi Naidu said she welcomed the Bank of England and UK government's stance, in "providing early certainty for in-bound branches".
Germany's Allianz also told Reuters it was applying for a branch licence for one of its units in Britain and after requests from brokers, Munich Re said it applied in March for UK licences as well, at a cost of "low double-digit million figure".
Insurers are also moving policies from London to new EU hubs, to ensure that customers can still pay premiums and receive payouts on cross-border contracts after Brexit Day. One-year policies taken out after March 29 this year are at risk.
Insurers may be implementing Brexit plans, but staff moves are so far modest, far fewer than the several thousand banking jobs expected to shift, according to a Reuters survey.
Thirteen insurers who gave details of job numbers in the survey in March said a total of 173 jobs would be created in or moved to the EU, mainly in Dublin and Luxembourg.
The BoE said in response to a Freedom of Information request from Reuters that it had received contingency plans from 170 insurers regulated by its Prudential Regulation Authority and 519 such responses from European Economic Area insurance firms and their home state regulators.
A lack of clarity on future trading links after transition is also hastening insurers' Brexit moves.
While Britain wants a bespoke trade agreement based on "mutual recognition", the bloc is pushing its existing system of market access for "third countries", or non-EU members.
"We are working under the assumption that Britain will be a normal third country in the future," Wenning said.