San Francisco is one of EOPT's key markets

EOPT is the largest US stock-market listed property company - controlling 580 buildings in 16 states as well as in Washington DC.

Specialising in corporate offices, it has annual rental income of $3.4bn.

Blackstone, founded in 1985, currently has $67bn of investments including almost $13bn in real estate.

The previous record private equity deal came in July when Bain Capital, Kohlberg Kravis Roberts and Merrill Lynch spent $33bn on Hospital Corporation of America.

Last week Clear Channel, the largest owner of radio stations in the US, was bought by a private equity consortium for $26bn.

Equity Office Properties Trust was founded by Sam Zell, a real estate tycoon who started by purchasing office property at distressed prices.

Last year he bought the Verizon building in New York for $515m.

The deal will include Blackstone assuming $16bn worth of debt held by EOPT.