LONDON (REUTERS) - The head of a major public inquiry into decades of child sex abuse in Britain resigned on Thursday (Aug 4) without giving a reason, the investigation's third leader to quit in the last two years.

The inquiry, which will last at least five years and cost about £18 million (S$31 million), was set up in July 2014 after a series of child sex abuse scandals dating back to the 1970s, some involving celebrities and politicians.

On Thursday, its chairwoman, New Zealand High Court Judge Lowell Goddard, quit without publicly explaining her decision.

"Dame Lowell Goddard wrote to me today to offer her resignation as chair of the independent inquiry into child sexual abuse and I have accepted," interior minister Amber Rudd said in a statement.

Goddard's appointment was seen as an attempt to give the inquiry a credible head without links to the British political establishment after her two predecessors resigned amid criticism over conflicts of interest.

Victims have accused politicians as well as the Catholic and Anglican Churches, councils and schools of failing to deal with abuse allegations.

In a number of cases, they said institutions had actively covered up cases at the behest of powerful establishment figures including senior lawmakers, spies and police officers.

"I want to assure everyone with an interest in the inquiry, particularly victims and survivors, that the work of the inquiry will continue without delay and a new chair will be appointed," Rudd said in her statement.