Citigroup, one of the world's largest companies, has warned its UK employees about the possibility of losing their jobs as it embarks on a sweeping restructuring that could affect hundreds of jobs in the country.

The bank, which has about 16,000 employees in Britain, said it was moving into the second phase of plans to rationalize its banking structure, according to a memo seen by Reuters.

"We anticipate that the reviews may lead to reductions in roles in some parts of the business and changes to some other roles," the note said.