Theresa May announces police will now investigate what happened to the document

Claims he was later visited by Special Branch who threw him against wall and took file

Document was passed to award-winning journalist Don Hale by late Labour minister Barbara Castle in 1980s

Crusading journalist Don Hale said officers from Special Branch seized documents naming MPs and peers with links to a paedophile group

Police are investigating whether Special Branch officers confiscated documents naming 16 MPs and peers with links to a paedophile group.

Campaigners fear they may have been destroyed in an establishment cover-up involving the predatory paedophile Cyril Smith – along with the missing ‘Dickens dossier’.

Theresa May announced the Metropolitan Police will investigate what happened to a file passed to award-winning journalist Don Hale by the late Labour minister Barbara Castle.

It allegedly included the identities of senior politicians who were actively promoting the Paedophile Information Exchange, which campaigned to legalise sex with children in the 1970s and 1980s.

Mr Hale said he was approached in the 1980s by Baroness Castle, who wanted him to investigate Westminster’s links to PIE.

Over a series of meetings, Lady Castle handed him a series of ‘extraordinary’ papers containing a list of 16 MPs and peers ‘allegedly involved in promoting the PIE network’, he told the BBC.

Mr Hale said that, after contacting the Home Office and some of the Parliamentarians on the list, he was visited by the Liberal MP Cyril Smith who, since his death, has been unmasked as a serial abuser of young boys.

Mr Hale said of Smith: ‘Suddenly he turned from Mr Nice Guy into Mr Nasty. He was very threatening and demanded I hand over all the documents to him immediately.’

He said that when he refused, Smith stormed out.

Mr Hale said he was then visited the next day by officers from Special Branch citing ‘national security’ who threw him against a wall and took the documents. The papers have not been seen since.

Mr Hale believes the files may have been even more important than those compiled by Tory MP Geoffrey Dickens, which have also disappeared.

Shadow home secretary Yvette Cooper said the allegations are ‘clearly immensely serious’. She added: ‘They must be investigated by someone.’

However, her party colleague John Mann questioned whether the Met was the right body to carry out the investigation, given its links to Special Branch.

Theresa May (pictured) announced the Metropolitan Police will investigate what happened to a file passed to award-winning journalist Don Hale by the late Labour minister Barbara Castle

Lady Castle, who died in 2002, is thought to have put together 30 pages of information about alleged attempts by the PIE to infiltrate government.

As well as key members of the Commons and Lords, she found about 30 prominent businessmen, public school teachers, scoutmasters and police officers had links to PIE.

Mr Hale went on to be a campaigning editor at the Matlock Mercury newspaper, known for the investigation that led to the freeing of Stephen Downing, wrongly jailed for the ‘Bakewell Tart’ murder.

In addition to his explosive claims, yesterday saw Mrs May come under more pressure to make progress in finding a new head for a wide-ranging inquiry into historic sex abuse.

Labour MPs are promoting human rights barrister Michael Mansfield QC’s case to be the new chairman, following the resignation of Fiona Woolf.

Campaigners fear the file may have been destroyed in an establishment cover-up involving the predatory paedophile Cyril Smith (pictured)

The National Association for People Abused in Childhood has also said it would support Mr Mansfield. However, one possible problem is that, in the late Seventies, Mr Mansfield had ties to the National Council for Civil Liberties, which granted the PIE ‘affiliate member’ status.

In 1978, he attended an NCCL meeting alongside Labour’s Harriet Harman, who has herself attracted huge controversy over the NCCL’s ties to PIE.