The national child sexual abuse inquiry will retain all of its investigations after an internal review concluded it should not be scaled back.

Chairwoman Professor Alexis Jay rejected suggestions that the remit of the probe is too broad to succeed – describing its scope as a “virtue”.

She said she plans to make recommendations in an interim report in 2018 and spoke of her determination to make “substantial progress” by 2020.

However, no final completion date has been given for what is the largest public inquiry ever established in the UK.

There have been suggestions that it could last for up to a decade.

On Friday, Professor Jay published a review of the troubled inquiry, which she announced in August after being named as its fourth head since it was launched in 2014.

Thirteen different investigative strands spanning several decades and examining a host of different institutions are currently being pursued.

There have been calls for the inquiry to be pared back, with former chairwoman Dame Lowell Goddard saying there was an “inherent problem” in its “sheer scale and size”.

However, a report setting out the findings of the review said the inquiry remains committed to pursuing each of the existing investigations as they play an “important part” in its task of examining institutional failure.

Prof Jay said: “There have been suggestions that the remit of the inquiry is too broad to succeed. I disagree. Its scope is a virtue, allowing it to recommend fundamental changes beyond the reach of an inquiry with a narrower remit.”

The inquiry is proposing changes to the scope and timing of the public hearing for the investigation into the institutional responses to allegations of sexual abuse involving the late Lord Janner of Braunstone.

In the Janner strand there will be a stronger focus on the institutions and it may not be necessary for a “finding of fact” to be made on the truth or otherwise of a specific allegation of child sexual abuse, according to a provisional determination published by the inquiry.

Submissions on the proposals are being sought before a final decision is taken, with “substantive” public hearings in the investigation unlikely to be held before 2018.

Lord Janner, who died in December, is alleged to have abused youngsters over a period spanning more than 30 years dating back to the 1950s. His family have always denied the claims.

Lord Janner's son, Daniel Janner QC, said he welcomed “the re-focus on alleged institutional failings”.

He added: “I also welcome the stepping back from the unfairness of making findings of fact in relation to these false allegations given my innocent late father is dead and cannot answer back.

“Complainants have issued civil proceedings during which they will be vigorously cross-examined, unlike in the inquiry.”

Elsewhere, the review:

* Finds that the strategic approach of the inquiry is right but its implementation has been too slow.

* Concludes that the inquiry's work needed “rebalancing” to make sure sufficient attention was paid to making recommendations for the future.

* Sets out measures to accelerate the progress of public hearings, with four planned for next year.

* Reveals that legal requests have been sent to hundreds of institutions, while 86,000 documents have been received so far.

* Says a new “internal governance structure” is in operation.

* Commits to an expanded programme of research and analysis for 2017/18, with a number of seminars planned.

Prof Jay said: “Every day, children face the horrors of sexual abuse and its consequences. I will do everything in my power to understand how this happened in order to help prevent it happening again.

“It is essential that we move the inquiry forward with renewed vigour. This review will provide a firm foundation in our work to protect children from abuse.”

The probe has been beset by problems, including a flurry of departures of senior figures in recent months.

A replacement for Ben Emmerson QC, who resigned as counsel to the inquiry at the end of September, is expected to be announced soon.