Two former IRA prisoners used threats in a bid to pocket a £150,000 pay-off from millions allegedly raised using the group's name, a court has heard.

Nick Mullen, 61, of west London, once jailed for being a "quartermaster" for an IRA active service unit, and Ronald McCartney, 56, of Belfast, who tried to kill three policeman in the mid-70s, warned a pair of businessman to hand over £150,000 "or face the consequences", it was claimed.

Their alleged victims, including one who used to provide security for Sinn Fein leader Gerry Adams during visits to Downing Street, were told they had a week to pay.

In the first threatening call the former security adviser was warned: "There has been an investigation and we know what you have been up to", London's Southwark Crown Court heard. He was then told he would receive a letter requiring him to follow instructions.

The businessman then rang several "well-placed" contacts, including Brighton bomber Pat McGee, so he could be to "put in the picture". After that he contacted the police.

Mark Heywood, prosecuting, said four days later letters dropped on the doormats of both men.

They not only bore the Irish Gaelic for the Provisional IRA - Oglaigh na h-Eireann - but were signed with the organisation's nom de guerre "P O'Neill".

It told them after a "prolonged and intensive investigation ... it had been established that both of you have used our organisation for personal gain".

It accused the former Sinn Fein supporter of abusing the "position of trust" he once held.

Mullen, of Birlington Mews, West Acton, and McCartney, 55, of Ross Road, Belfast, deny two counts of conspiracy to blackmail between January 1 and April 16, 2008.