If an application is successful and submitted before 9.30am, the visa will be ready for collection by 5.30pm the same day in New Delhi or by 6.30pm the same day in Mumbai.
The service had been announced by British Prime Minister David Cameron during his visit to the country in February this year.
"I am delighted to see the visa service being launched in India before any other country. We expect this new service to be particularly useful for business," said Sir James Bevan, the British high commissioner to India, adding that UK and India are already on course to double bilateral trade by 2015.
India remains the UK's biggest visa operation in the world, processing around 400,000 applications every year.