Chancellor Philip Hammond has hailed a new Facebook office as a "sign of confidence" in Britain.
The HQ near London's Oxford Street will lead to the creation of 800 new jobs, more than half of which will be in engineering.
It will open later today.
Facebook opened its first office in London 10 years ago and has said it expects its British workforce to reach 2,300 by the end of next year.
Mr Hammond said: "It's a sign of confidence in our country that innovative companies like Facebook invest here, and it's terrific news that they will be hiring 800 more highly skilled workers next year."
London Mayor Sadiq Khan said Facebook's commitment showed the city is "at the forefront of global innovation".
The new headquarters was announced last year, shortly after Google revealed it would be building a new office in the city to accommodate more than 7,000 workers.
Nicola Mendelsohn, Facebook's vice president for Europe, the Middle East and Asia, said: "The UK's flourishing entrepreneurial ecosystem and international reputation for engineering excellence makes it one of the best places in the world to build a tech company.
"And we've built our company here – this country has been a huge part of Facebook's story over the past decade, and I look forward to continuing our work to achieve our mission of bringing the world closer together."
The seven-storey office space is 23,000 square metres and was designed by architect Frank Gehry.
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It will also house Facebook's first in-house start-up incubator, called LDN_LAB, designed to help kick start fledgling British digital businesses.
As well as engineers, there will also be developers, marketing and sales teams.
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