Luqman Arnold's activist financial services vehicle Olivant has spent almost £500 million doubling its stake in Swiss banking giant UBS in the past month.

Olivant, which was one of the last two potential bidders for Northern Rock, now holds a 2.5% stake worth Swfr1.9 billion (£930 million) and is campaigning for major changes.

Arnold, a former chief executive of UBS investment bank, has been critical of its decision to replace chairman Marcel Ospel in the wake of Swfr37 billion of subprime losses with the bank's leading lawyer, Peter Kurer. Earlier this year, Arnold asked "He's been part of this group for seven years. How can he be the person who comes in and says 'I'm the solution?'"

But today Olivant said it was "engaged in a constructive dialogue with the chairman and certain members of the board and senior managers of UBS."

It added that it "welcomes and supports a number of the recent decisions and actions being taken by the board" and said it had fully supported the bank's Swfr16 billion rights issue, completed last week.

But Olivant made it clear it would keep up the pressure, saying it trusted that the UBS board "understands the urgent need to clarify the future strategy of the company". It added: "This is a prerequisite to being able to attract and retain leading talent and to communicate effectively strategic direction and momentum to all stakeholders."