Barclays of Britain plans to name Robert E. Diamond Jr. as its chief executive as soon as Tuesday, two people briefed on the appointment said Monday. Mr. Diamond, 59, helped turn Barclays into a global brand with the acquisition of Lehman Brothers' operation in the United States. He was also responsible for much of the bank's international and financial growth in the last decade. He would succeed Barclays' current chief, John Varley. Mr. Varley, 54, would leave Barclays at the end of March after a transition period, these people said. They were not allowed to discuss the matter publicly before the formal announcement. Jerry del Missier and Rich Ricci, co-chief executives for corporate and investment banking at the bank, are to replace Mr. Diamond at the top of Barclays Capital, the securities unit, one person said.