Jefferson Azevedo also pleaded guilty to planting a hoax bomb on a bridge over the A27 in Havant, Hampshire.

An investigation by Hampshire Police discovered more than 150 letters had been sent to politicians and other high profile figures between 2003 and 2007.

Among Azevedo's victims were former prime minister Tony Blair, the Attorney General's office, Portsmouth Royal Naval Dockyard and a primary school in Havant.

Other organisations to receive threatening letters included a Portsmouth curry house, offices of the Voluntary Service Overseas organisation and Slough offices of mobile phone company O2.

Azevedo appeared at Southwark Crown Court, in London, where he entered guilty pleas to nine charges under counter-terrorism legislation, nine further charges of sending letters containing racist abuse and another relating to the explosive device.

He also asked for a further 140 offences to be taken into consideration.

The 45-year-old, of Langley Road, Portsmouth, was remanded in custody to appear at the same court on June 13 when he will be sentenced.