* Activist Bramson's Sherborne vehicle owns 4.2 pct of 3i
* 3i says it has "wasted no time" to restructure
* 3i shares have risen 65 pct since mid-2012
LONDON, March 1 (Reuters) - Private equity group 3i defended its turnaround strategy on Friday after it emerged that activist investor Edward Bramson had increased his stake in the company.
Bramson, known for forcing through management shake-ups at his target companies, has bought 4.2 percent of 3i through his investment vehicle Sherborne, at a cost of 105 million pounds ($159 million), Sherborne said in a statement.
In January 3i revealed Sherborne and its broker Jefferies had been buying shares, and in total owned a stake of around 1.6 percent.
3i has been on a drive to boost its performance since appointing a new chief executive in May last year, cutting jobs and retrenching to its northern European roots in an effort to win over shareholders frustrated at poor share price performance and weak results from its buyout business.
Shares in the group have gained almost 65 percent since chief executive Simon Borrows launched his restructuring plan.
"We have wasted no time in implementing the significant organisational and cultural changes that were needed," Borrows said in a statement.
"We are nine months into the first phase of restructuring at 3i and our focus is now moving to the next phase...generating value and sustainable performance for our key stakeholders."
A spokesman for Sherborne, which said in Friday's statement it may sell part or all of its positions in 3i, or add to it, declined to comment further.
Last year Sherborne raised 207 million pounds with the aim of investing in an unspecified listed company it considered undervalued.
Sherborne said in a November statement its "investment objective is to realise capital growth from investment in a target company", and that it would "only invest in one target company at a time."
Media-shy Bramson has a history of taking on the managements of companies he considers to be underperforming.
In 2011 he led a boardroom coup at British fund firm F&C Asset Management, ousting the chairman and pursuing a root-and-branch shake-up of the 144-year old company after years of share price underperformance.