The loss compares with a profit of £628m a year earlier, and reflects a total of £6.7bn in claims absorbed during the first half, making it the costliest six-month period in Lloyd's 323-year history.
Many insurers have reported steep half-year losses after an unprecedented run of natural disasters which also included an earthquake in New Zealand and heavy flooding in Australia.
This year already ranks as the second most destructive on record for catastrophe losses after 2005, with insurers absorbing $70bn (£44.5bn) in claims in the first six months alone, according to Swiss Re, the world's No. 2 reinsurer.
Last month, Catlin, operator of the biggest Lloyd's syndicate, sank to a first half loss of $201m, while rivals Amlin and Hiscox unveiled deficits of £192.3m and £85.6m respectively