Call centres in India brought in extra staff and asked some to work overtime to answer a flood of inquiries to Britain's National Rail after a series of deadly bomb blasts hit London's transport network Thursday, officials said.

A significant proportion of National Rail's queries are outsourced to Indian contact service providers.

"Everyone is working overtime and no one is taking a break," Ashok Dhawan, chief operating officer of Intelenet Global Services, told Dow Jones Newswires.

Intelenet employs some 5,000 people at its call centre in Mumbai, catering mostly to clients in Britain.

Dhawan said the number of calls to Intelenet centres doubled after near-simultaneous explosions rocked London during Thursday's morning rush hour. He refused to disclose the number of calls handled by the Mumbai centre on a normal day, citing contractual obligations.

ClientLogic, a global contact service provider with a call centre in the southern city of Bangalore, which also deals with National Rail inquiries, said it increased staffing level by more than 15 per cent.

"We have beefed up the team to make sure that we can handle the volume," chief operating officer Neeraj Khan told Dow Jones.

Scores of Western companies farm out software development, engineering design and routine office functions such as answering customer calls to India and other countries where skilled workers are plentiful and wages low.

India controls 44 per cent of the global offshore outsourcing market for software and back-office services, with annual revenues of $17.2 billion (�14.07 billion), according to the nation's main infotech trade body.