The brothers were reported to have been non-doms, a status that let them pay U.K. tax only on income earned or brought into the U.K. That was until Britain abolished it last year—a change that also brought long-term residents’ worldwide assets within reach of Britain’s 40% inheritance tax.

Monaco, meanwhile, levies no income, capital gains, or inheritance tax. The brothers’ combined fortune is estimated at almost £28 billion, and Bloomberg’s Billionaires Index values David Reuben alone at $13.1 billion.

Across the globe, a similar drama is playing out in California. Britain’s exodus follows a tax regime already dismantled, while California’s is unfolding ahead of one voters are waiting to approve. 

Early voting opened this week on Proposition 40, a one-time 5% levy on the state’s roughly 200 billionaires. Google cofounders, venture capitalists, and a Hollywood director have already decamped for Miami, Las Vegas, Austin, and New York, and didn’t wait for the result. 

Reuben’s spokesman confirmed the move but declined to comment further on why he made it. Simon Reuben has lived in Monaco for about 40 years after moving there for health reasons.

Reuben’s move comes about 18 months after the U.K. scrapped its “non-domicile” tax status, which for more than two centuries let wealthy residents with permanent homes abroad avoid U.K. tax on overseas income. The new residence-based system took effect April 6, 2025, Fortune previously reported, and tax advisors warned then that the biggest sticking point was the prospect of Britain’s 40% inheritance tax reaching long-term settlers’ global assets.

Since then, the departures have piled up. More than a dozen billionaires worth a combined £120 billion have quit the U.K. in the two years since Labour took power, according to a new analysis of Bloomberg Billionaires Index data reported by the Evening Standard. That tally doesn’t include Reuben. Chancellor John Healey is now under pressure to raise revenue in his first budget on Oct. 28.

California’s own billionaire tax battle

Sen. Bernie Sanders (I-Vt.) and Rep. Ro Khanna (D-Calif.) rallied for Prop 40 in San Francisco on Saturday, with Sanders calling it the most important ballot initiative in the country. Gov. Gavin Newsom is a critic, warning that billionaires can simply move to another state to avoid it.

California already has early returns on that argument. Six of the state’s 214 billionaires were widely reported to have left before the initiative’s Jan. 1, 2026, residency cutoff, including Larry Page, Sergey Brin, Peter Thiel, car-loan magnate Don Hankey, former Uber CEO Travis Kalanick, and Steven Spielberg. By Fortune‘s Marco Quiroz-Gutierrez’s most recent calculation, Page, Brin, Thiel and Kalanick alone would have owed about $29 billion—more than a fourth of the measure’s projected $100 billion haul.

Brin has since donated $102 million to Building a Better California, the group leading the opposition, according to campaign filings cited by Fortune’s Sasha Rogelberg. He could owe roughly $13 billion if the tax passes. Opponents have spent more than $187 million against about $32 million in support, and two billionaire-backed countermeasures, Propositions 41 and 42, would nullify Prop 40 if either draws more votes.

To be sure, not every billionaire is running away. Nvidia CEO Jensen Huang told Bloomberg Television he hadn’t thought about the tax once and would pay whatever Silicon Valley levied. 

“We chose to live in Silicon Valley and whatever taxes they would like to apply, so be it,” Huang said. “I’m perfectly fine with it, it never crossed my mind once.”