SCOTLAND'S police ­watchdog blocked a range of critics from giving evidence to its high-profile investigation into its policy of stop and search.

The Scottish Police Authority (SPA) intended to ask human rights and equality groups to be witnesses during the inquiry, but dropped them and only contacted law and order organisations for their views.

The SPA, set up to scrutinise Police Scotland, launched a review into stop and search last year amid mounting concern about the policy.

Frisks are a signature policy of the single force's chief constable, Sir Stephen House, but fears have been expressed about the vast number of searches conducted, and the impact on children.

The majority of searches are also termed "consensual", meaning they have no basis in law.

However, the SPA's final report was deemed to be a damp squib by critics, who believe the watchdog avoided many of the policy's most controversial aspects.

The SPA's original "scoping" ­document promised a far-reaching inquiry. Supporters of the policy - Police Scotland, the Scottish Police Federation, and the Association of Scottish Police Superintendents - were on the list of witnesses, but so were bodies that had voiced reservations. These included Amnesty International, the Equality and Human Rights Commission and the Scottish Human Rights Commission, which recently reported concerns about stop and search to the United Nations.

Scotland's Commissioner for Children and Young People, Tam Baillie, another critic of the policy, was also on the list, as was Dr Genevieve Lennon, a Chancellor's Fellow at Strathclyde University who has written about suspicionless searches.

However, after the review was launched, none of these individuals or organisations was asked to give evidence. A source close to the review said the omissions resulted in a "one-sided" picture emerging from the review.

The SPA also watered down its final report. In the draft, it revealed the search rate was nine times higher in Scotland than in the area covered by the New York Police Department. However, the statistics were later ditched.

Critical interviews with police officers were also not published alongside the final report, but instead surfaced days later on the SPA website with little fanfare.

Scottish Liberal Democrat justice spokeswoman Alison McInnes said: "A proper review should have invited critical friends to comment. The SPA should explain why they changed their minds and did not seek counsel from these leading human rights voices as part of their review into stop and search.

"Attempts to stifle criticism of this controversial policy should be roundly condemned by people in Scotland, who rightly expect human rights to be front and centre of our approach to justice policy."

A spokesperson for the SPA said: "The SPA is confident that the approach it took to scrutinising and reviewing the available data, evidence, and views within policing on the policy and practice of the stop and search tactic was intelligent, informed and independent.

"It led us to make 12 searching recommendations for improvement that have already been influential in informing changes to policy and practice within Police Scotland."