The Asian Development Bank on Friday awarded its biggest loan for an infrastructure project, almost one billion dollars, to help finance Mumbai's ambitious underground metro network.

The ADB said it would lend India $926 million to help fund two lines on the teeming financial capital's first subway network.

A bank statement said the lines would 'ease the distress of millions of commuters each day and help provide a cleaner, less congested city. "

It described it as "the single largest infrastructure project loan in ADB history". The Indian government has announced plans to build 12 metro lines totaling 276 kilometers (171 miles) in Mumbai.

Backers say the scheme is essential to solve the city's traffic woes and ease the load on its creaking surface railway lines, but the project has been mired in controversy. The main line, called Metro 3, has faced considerable opposition in court, delaying its completion to December 2021.

Environmentalists tried unsuccessfully to stop the felling of thousands of trees and are unhappy at plans to build a metro car shed in an urban forest.

Thousands of Parsis, meanwhile, called for the route to be changed so trains do not pass under fire temples where Zoroastrians worship.