The shifting of UK financial services jobs to developing countries such as India and China has saved the sector about £1.5bn a year, a study suggests.
Outsourcing services to India is popular with UK financial firms
Accountants Deloitte said the number of financial jobs going overseas over the past four years had increased 18-fold.
More than 75% of major financial institutions have operations overseas, compared with fewer than 10% in 2001.
But trade union Unite said an effective case for offshoring had not been made, and pointed to growing staff turnover.
It says that many companies are having to retrain an entire workforce over the course of a year and that wages are rising.
China v India
Offshoring has spread across nearly all business functions, with significant growth around transaction processing, finance and human resources.
India has kept the top spot for firms looking to move processes overseas, with about two thirds of global offshored staff employed there.
But it is in danger of losing its crown to China, with one third of financial institutions now having back-office - mainly IT - processes there.
Some 200 million Chinese people are currently learning English, providing a potential pool of skilled workers that may compete with India in coming years.
Chris Gentle, associated partner for financial services at Deloitte and author of the study, said: "Financial institutions need to re-engineer business processes, or risk simply transferring offshore the legacy inefficiencies of older, onshore processes."
The typical financial services firm now has 6% of its staff outside the host country, with the proportion having doubled in the past year.
However David Fleming, national officer at Unite, said the human and social costs of moving jobs "have been absolutely huge and customer dissatisfaction is widespread".
He added: "Unite still believes that organisations are overlooking the complexities of offshoring and are still failing to make a sound business case for exporting work overseas."