SIR Anthony Blunt, the surveyor of the Queen's pictures unmasked in 1979 as a Soviet spy, may have been an illegitimate son of George V, according to a controversial new book on the royal family.

War of the Windsors - A Century of Unconstitutional Monarchy, published today by Mainstream in Edinburgh, also claims that Lord Mountbatten was approached to be the figurehead leader of a coup d'etat to depose the Wilson government in 1968 amid fears that the country was in ''economic and moral decline''.

The authors, Stephen Prior, a former British intelligence officer, Clive Prince, Lynne Picknett, and Robert Brydon, also allege that the royal family were involved in machinations to seek an accommodation with Hitler during the war and thereby ''guilty of treason of the highest order''.

War of the Windsors is Picknett and Prince's fifth major investigative work after Double Standards, the Rudolf Hess Cover-up, the Turin Shroud: In Whose Image, The Templar Revelations, and The Stargate Conspiracy.

It follows hard on the heels of the acquittal of Paul Burrell, the former royal butler, after a three-week trial at the Old Bailey in London.

Mr Prior said: ''The case of Paul Burrell may well prove to be the issue that finally leads to a radical reappraisal and overhaul of Britain's outmoded constitutional monarchy.

''The Burrell revelations, however, are merely the tip of an iceberg of royal sleaze, duplicity, and outright constitutional manipulation and malpractice - as War of the Windsors shows.''

According to Phoenix, a source of the authors who knew Blunt and worked as an officer for M15, Blunt confessed to being a Soviet spy in 1964 after being under suspicion for at least a decade.

Mysteriously, however, no action had been taken against him. Blunt was not publicly unmasked until 1979, despite his confession 15 years previously. He was allowed to remain as surveyor of the Queen's pictures until his retirement in 1972.

Mr Prior said: ''Why was his treason not punished? Despite the 'blackmail' he no doubt possessed on the royal family through his knowledge of secret correspondence he uncovered in post-war Germany relating to royal peace overtures, there is the strong possibility he may have been of royal blood himself.''

War of the Windsors details how Blunt, ostensibly the son of a humble vicar, may actually have been a bastard son of George V.

Incredible as it sounds, the available evidence is intriguing. George V is known to have enjoyed the favours of mistresses. It is conceivable that one of these mistresses may have been Blunt's mother

The book features photographs of Blunt, the Duke of Kent, and the Duke of Windsor - all born in the same generation. Although not conclusive of a blood relationship, the resemblances are remarkable.

Blunt was born in September 1907, the youngest of three sons. His father was Reverend Stanley Blunt, vicar of the village of Ham, and his mother Hilda Masters. They married in 1900 and in 1906, a year before Anthony was born. Blunt died in 1983.

Hilda Masters knew Queen Mary before she became Queen - they used to go blackberry-picking together - and the two women remained friends for the rest of their lives. The book says, of George V, husband of Mary: ''Even this allegedly most uxorious of royal husbands was known to have mistresses - at the seaside resort of Bognor, for example.

''Like many Victorian ladies, Mary may have welcomed the reprieve from the horror of sexual intimacy, even if it meant her great friend took her place in her husband's bed.''

The book alleges the centrepiece of the Mountbatten plot lay in persuading the Queen to dismiss the government, and then, with the backing of the armed forces set up a government of national unity.

The coup attempt came to nothing, but it now appears that there was a faction within MI5 that supported the idea because it believed Harold Wilson, the prime minister, was a Soviet agent.

It emerged in 1987 that the Cunard shipping line had been approached by senior figures in the army and security services in preparation for the liner QE2 to be requisitioned as a detention centre for the cabinet.

Last night, a spokesman for Buckingham Palace said: ''We are not prepared to give any comment which would dignify the book.''