U.S.-based fund manager Harris Associates has built a $1 billion stake in troubled Swiss investment bank UBS, the Sunday Times said in an unsourced report.

UBS, Switzerland's largest bank, unexpectedly replaced Chief Executive Peter Wuffli with his deputy, Marcel Rohner, earlier this month following growing pressure from critics to bolster results and perhaps break up the company by getting out of investment banking.

Speculation of a possible break-up at UBS has escalated in recent months following a proposed three-way carve-up of ABN AMRO by a group led by Royal Bank of Scotland.

The Sunday Times report said Chicago-based Harris Associates had built a stake representing about 1 percent of UBS over the past three months.

Britain's Lansdowne Partners has also built a significant stake, the paper added.

UBS declined to comment.