Although BMC had high hopes of the Princess R, and public reaction was favourable at first, its attraction soon faded away, production fell to a trickle in the latter years, and it eventually died away, a commercial and marketing gamble which failed.
In the early 1960s, BMC was in a very expansionary mood and spent some time talking to Rolls-Royce in general terms about merging some interests. Although these talks were eventually abandoned, the single concrete result was that the prestigious Rolls-Royce company developed a special version of their six-cylinder engine, which they called the FB60 type, and this was then offered by BMC in a much revised Princess saloon. Because it was a 3,909cc unit, the new car's title was 4-Litre Princess R, and rumours persisted that the R was meant to stand for Royal, but that the use of this adjective was frowned on in court circles.
The basic design, style and structure of the Princess R was still that of the Princess 3-Litre (which meant that it was still related to the Austin A110), but the design had been reworked from stem to stern to justify a much higher price and to make it more exclusive and worth of the Rolls-Royce engine.
The engine itself was an alloy-block, short-stroke version of the familiar six-cylinder Rolls-Royce unit, which was used not only in Rolls-Royce and Bentley passenger cars up to 1959, but in many military vehicles. With a quoted peak of 175bhp it was much more powerful than the superseded 3-litre BMC C-Series engine, though this was never backed up by flashing acceleration figures or a dramatically higher top speed.
Automatic transmission was standard, the suspension was virtually the same as before except for a massive new front cross-member, and there was even more attention to sound-deadening.
The bodyshell was changed, in detail, in many ways. Compared with the A110, there was a new roof panel (offering more headroom), no peaks over the front and rear windows, a more upright rear window in any case, and the rear end was smoothed out by eliminating the fins and providing horizontal tail-lamp clusters. However, the most important change of all was to the price - which was £1,994, compared with £1,474 for the 3-Litre in the UK market.
Sales took off with a rush, but once the word got around that the engine was wasted on the rest of the car, demand slumped. Even with Vanden Plas trying all they knew to ensure top build quality, BMC could not sustain interest in the car. Sales had been expected to exceed 100 cars a week, but such figures were not realized and production was down to 200 cars a year by 1967. The Princess R was quietly withdrawn from production at the beginning of 1968, just before British Leyland would surely have killed it off.
The Vanden Plas factory at Kingsbury closed down in 1979, the last Vanden Plas car was produced at Abingdon in 1980, and more recently the name has been applied by Austin-Rover to versions of Austin and Rover cars.