Seems to affect those made in the last two years.
The BBC TV consumer programme was alerted to the problem by a Kent-based commercial cleaning company, Town & Country Cleaners, which has three of the Citroen vehicles in its fleet.
The firm only discovered it when one of their drivers travelling at 70mph (112kmph) on a motorway found his car suddenly screeching to a halt.
The programme has since tested two more Picasso C3s on sale at dealerships in south east England - both of which have the same flaw. The model has been on sale in the UK for the last two years.
Motor engineer expert Mark Brown, who examined the cars, found that the braking mechanism underneath the passenger floor was easy to press - as it was only covered with a thin piece of felt.