Kolkata grew steadily until 1756, when Siraj-ud-daula, the nawab of Murshidabad, attacked the town. Most of the British inhabitants escaped, but those captured were packed into an underground cellar where, during the night, most of them suffocated in what became known as 'the black hole of Calcutta'. In 1757 the British, under Clive of India, re-took the city and made peace with the nawab.

A stronger fort (Fort William) was built in Kolkata and the town became British India's capital. Over the following centuries, the British created a facsimile of London on the banks of the Hooghly, with parks, gardens, parade grounds and wide boulevards lined with colonial mansions, many of which still stand today. Kolkata's Indian population was relegated to the bustees - sprawling slums extending for miles around the British enclave. This unfair segregation sowed the seeds for rebellion. In the 19th century, Bengal became an important centre in the struggle for Indian independence, leading the British to transfer the capital to Delhi in 1911.