The independent inquiry into child sexual abuse is in danger of "taking years to confirm what we already know", a former top prosecutor has warned.

Lord Macdonald QC said the probe's "overwhelming focus on the past" is "ballooning out of control" and called for its terms of reference to be rewritten.

His comments come after former head of the inquiry Dame Lowell Goddard argued that it should be overhauled.

Her replacement as chairwoman Alexis Jay said she has already started a wide-ranging internal review of the inquiry's ways of working - but ruled out revisions of the terms of reference or restrictions on its scope.

Lord Macdonald of River Glaven QC, who was director of public prosecutions from 2003 to 2008, told the Press Association: "The inquiry's overwhelming focus on the past is ballooning out of control, in real danger of taking years to confirm what we already know.

"It has become an investigation into 60 years of everything everywhere, and its scope is now the enemy of a useful outcome.

"We have to learn from the past without obsessing over it to the exclusion of making a better future.

"So its terms of reference should be urgently rewritten to focus on policy and legal reforms that will best protect children in the future."

Last month Dame Lowell became the third chairwoman to quit the inquiry, which has been beset by problems since it was launched in 2014.

In written evidence to the Commons Home Affairs Committee, the New Zealand high court judge said: "With the benefit of hindsight - or more realistically - the benefit of experience, it is clear there is an inherent problem in the sheer scale and size of the inquiry (which its budget does not match) and therefore in its manageability."

She added: "I have recommended in my report to the Home Secretary that my departure provides a timely opportunity to undertake a complete review of the inquiry in its present form, with a view to remodelling it and recalibrating its emphasis more towards current events and thus focusing major attention on the present and future protection of children."

The probe was given a budget of £17.9 million for 2015/16 and has been described as the most ambitious public inquiry ever in England and Wales.

It was earmarked to take five years, but there have been suggestions it could run for as long as a decade.

Prof Jay said: "As I said when I was appointed, the panel intends to ensure that the inquiry undertakes its work with pace, confidence and clarity.

"I want to reassure victims and survivors that the panel will not be seeking any revision of the inquiry's terms of reference or introducing any new restrictions on its scope."

She added: "To ensure that the inquiry can meet the challenges it faces, I have already initiated a wide-ranging internal review of the inquiry's ways of working and we are currently looking at different approaches to evaluating the information we receive.

"We are committed to maintaining efficiency and effectiveness so that the Inquiry can be concluded within a reasonable time and with a constant eye to the cost to the public purse."

Asked whether Theresa May agreed there should be a rethink of the inquiry, the Prime Minister's official spokeswoman said: "Dame (Lowell) Goddard resigned over a month ago. The Government signalled its commitment to the need to tackle these challenges with the swift appointment of Alexis Jay.

"That is because we believe the inquiry is absolutely vital and we remain committed to doing it."

Home Secretary Amber Rudd will be questioned about the inquiry when she appears before MPs on Wednesday.