Google co-founder Sergey Brin has donated a further $20 million to Build a Better California, an anti-tax campaign group opposing the state's proposed "billionaire tax," bringing his total spending against the measure to over $100 million. The move highlights how some of Silicon Valley's wealthiest figures are mobilising to block a levy that would target around 200 of California's richest residents, at a time when the state faces significant healthcare funding shortfalls.

Prop 40, which Californians will vote on in November, would impose a one-off 5% tax on billionaires' net worth, raising funds primarily for healthcare programmes as the state's Medicaid system braces for the loss of up to $30 billion in federal funding. Brin, worth an estimated $267 billion, would reportedly owe around $13.3 billion under the measure; the group he funds has proposed rival ballot measures to effectively block it. Other tech figures including Mark Zuckerberg, Travis Kalanick, Peter Thiel and Larry Page have reportedly relocated away from California, while Governor Gavin Newsom has criticised the tax's potential economic fallout but backed a national billionaire tax instead. Not all billionaires oppose it, however — Nvidia's Jensen Huang has said he is "perfectly fine" paying his roughly $8 billion share.