Senior cabinet ministers in Margaret Thatcher's Conservative government were supplied with underage boys for sex parties, ex-Tory activist Anthony Gilberthorpe has claimed.

In an interview with the Sunday Mirror, Gilberthorpe says he was asked to find underage rent boys for a private sex party at a hotel.

After 30 years, the 52-year-old has come forward, saying he was acting on the orders of some of the most powerful and senior members of Thatcher's government.

Many of the claims centre around conferences at Blackpool and Brighton in the 1980s, at which boys as young as 15 were given alcohol and cocaine before having sex with the politicians.

The former Tory activist says he was told to find the "youngest and prettiest" boys for several cabinet ministers as the "entertainment".

Those alleged to be present at the parties included Sir Keith Joseph (Secretary of State for Education and Science), Rhodes Boyson (Minister of State for Northern Ireland), Dr Alistair Smith and Michael Havers. All are now deceased.

Gilberthorpe claims that at the 1983 Blackpool conference he was asked by Dr Alistair Smith, the Tory Party Chairman in Scotland, to arrange for young rent boys to have sex.

"Dr Smith, who I looked up to at the time and was the most important Tory in Scotland, told me to go and fetch some 'entertainment', which was code for young boys, and handed me a handful of bank notes. There was about £120."

He said: "It was a norm and an open secret that these older members of the Tory party, like Dr Alistair Smith, paid for young men to join them at sex parties.

"There was a well known and used cruising area close to the Imperial Hotel, which was a conference hotel."

Gilberthorpe said he witnessed two senior Tories having sex with the boys. He said: "A couple of other MPs were in the room. I can clearly remember seeing one politician, who is now a serving Tory MP, standing there and watching."

Gilberthorpe also claims that two years earlier in 1981 he saw Sir Michael Havers – then Attorney General – at a swimming pool party at the Tory conference in Blackpool, where underage boys were encouraged to perform sex acts on several politicians.

Two inquiries are to be held into alleged child sex abuse involving MPs as well as other institutions such as the NHS, the Church and the BBC.

Gilberthorpe has come forward to expose the child abuse cover-up. "I am prepared to speak to the inquiry. I believe I am a key witness."