Luqman Arnold, the rebel shareholder pushing for boardroom reform at UBS, has bought about another £150 million of shares in the beleaguered Swiss banking group.

Purchases over the past two weeks have lifted the stake of Olivant, Mr Arnold's investment boutique, from 0.7 per cent of UBS to 1.1 per cent.

Mr Arnold revealed the stakebuilding yesterday and expressed himself pleased after meeting two UBS independent directors - Sergio Machionne, the head of Fiat and the deputy chairman of UBS, and Peter Voser, the finance director of Shell - on Thursday.

Mr Arnold is campaigning for UBS to launch a search for a new chairman immediately, arguing that Peter Kurer, the chairman-elect, is not up to the job of managing an organisation of 85,000 people.

Mr Kurer is due to replace Marcel Ospel at the annual meeting next Wednesday.

Mr Ospel, who is credited with building UBS into one of the biggest banks in Europe, announced plans to leave this month after admitting sub-prime and related losses of $37 billion (£18.7 billion).

Mr Arnold said yesterday: “We had an open and constructive discussion and they gave their commitment that they are taking all of our views into consideration as they prepare for the AGM. I have the highest regard for both men and look forward to hearing what measures the supervisory board will announce at the AGM.”

Olivant is pushing for UBS to modernise its boardroom, to embark on asset sales of its fund management unit and Brazilian and Australasian offshoots and to consider a more radical break-up.

It has criticised the planned appointment of Mr Kurer, the in-house lawyer at UBS, because of his lack of experience of managing large organisations and his lack of banking, credit and risk experience.

Mr Arnold, who hired Mr Kurer as in-house counsel when he was chief executive of UBS, said: “We can see no rational reason for the supervisory board to recommend his nomination, other than to exclude an outsider.”

Mr Arnold and Olivant mounted a rescue offer for Northern Rock but pulled out shortly before the decision to nationalise the ailing bank, arguing that the Government had been too tough in negotiations.

UBS has been rocked by the scale of its losses - the worst of any bank in the world - and has been forced to embark on a £7.5billion rights issue to shore up its balance sheet.

It is also poised to announce a round of job losses in its investment banking unit, with the London-based operations expected to shed about 900 people in early May.

UBS declined to comment on the discussions.

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