Cameron took time out from his general election campaign to record birthday wishes for the TV presenter dressed in the Stig's famous white racing suit and helmet.
"Cameron's video got a big laugh. Who knows, maybe he really is The Stig?" a source told The Sun.
Over 100 friends attended Clarkson's party, which was held in a marquee at his Cotswolds home. Guests included his Top Gear co-stars James May and Richard Hammond and TV comics Dom Joly, Paul Whitehouse and Harry Enfield.
Clarkson has previously caused controversy by insulting Prime Minister Gordon Brown during an interview in Australia. He ranted about Brown a second time in a non-filmed warm-up segment for Top Gear.