Oswald Mosley, Ramsay MacDonald and the pornographic blackmail letters

In 1929 Ramsay MacDonald, embarking on his second term as prime minister, was threatened with blackmail by an ex-lover. A Viennese socialite called Frau Forster, newly ruined in the Crash, threatened to publish his “pornographic” letters to her if he didn’t stump up some hush money. That, anyway, was the story as related in old age by Sir Oswald Mosley, who claimed to have dealt with the problem at MacDonald’s behest by sending “the old girl” packing with threats of imprisonment (Mosley was then a Labour MP, and one of MacDonald’s ministers). He also claimed that Forster resumed her blackmail attempt the following year – by which time he and MacDonald had fallen out – and hinted that this time MacDonald set the intelligence services on her tail.

From this scanty material, plus a couple of corroborative sources, Andrew Williams has fashioned a wickedly entertaining tale of political chicanery. It doesn’t seem much of a scandal by today’s standards: MacDonald had long been widowed by the time of the alleged affair. But Williams makes it clear that the publicly righteous MacDonald had sold himself to the electorate on his integrity (always a tactic that risks making you a hostage to fortune, as Sir Keir Starmer’s travails over Beergate have shown) and would have felt the need to get the letters back at any cost – even getting into bed with a toff-heavy intelligence service that despised him as the thin end of the Red wedge.

Williams, a former Newsnight and Panorama producer, is a master at making the connections that allow him to fill in the interstices of history, as he has shown in thrillers with backdrops ranging from the assassination of Tsar Alexander II to the subfusc activities of the fledgling British intelligence services in the First World War. His thesis here is that “a cabal of former and serving intelligence officers” – who had already played a part in MacDonald’s defeat in the 1924 election with such propaganda wheezes as the forged Zinoviev letter – would not have scrupled to use MacDonald’s predicament against him.

As different factions within MI5 and MI6 vie to get hold of the ­letters, Williams creates a delicious imbroglio involving such real-life figures as William Joyce (the future Lord Haw-Haw), pioneering spymaster Maxwell Knight, and senior Labour politician Jimmy Thomas (described as somebody “who dropped his aitches as deliberately as a beautiful woman put on her make-up”). The most memorable character, though, aside from the hapless MacDonald and the mercurial Mosley, is fictional: a working-class Great War veteran and spy called Dick Stewart, whose survivor’s guilt fills him with enough self-loathing to enable him to take on missions incompatible with his principles. Commissioned by more than one paymaster to appropriate the letters, he has to decide whose side he’s on; and it’s his faltering journey towards decency – while his betters all seem to be heading in the other direction – that gives this entertainingly unedifying saga some emotional heft.

The Prime Minister's Affair is published by Hodder & Stoughton at £20. To order your copy for £16.99 call 0844 871 1514 or visit Telegraph Books