Just days before the 150th anniversary of his birth on March 7, two hitherto unknown paintings by Piet Mondrian have surfaced which can now be added to the artist’s official, and still expanding, catalogue.

The paintings of Nicolaas and Elisabeth, two of his friend Cees Bergman’s children, date from 1907 and 1908 when Mondrian supplemented his income by painting portraits, often of his friends and family

The portraits of Bergman’s two other children Caroline and Cornelis were known already but it was only through the detective work of Nick Draaijer who was researching the history of his family, that it became clear the other children also sat for Mondrian.

‘As a child was always fascinated by the portrait on the wall of my grandmother’s house, of my great grandmother Elisabeth Bergman- Cavalini, Cees Bergman’s wife,’ Draaijer told broadcaster NOS.

That portrait too, was by Mondrian and in the course of his investigation Draaijer found his family were still in possession of the portraits of Nicolaas and Elisabeth as well.

The Dutch institute of art history RKD has confirmed the paintings are by Mondrian, even without having examined them. ‘I haven’t seen the actual paintings but I would swear blind that they are by Mondrian,’ Mondrian expert Wietse Coppes told the broadcaster. ‘Based on the photographs and the fact Mondrian painted the other two children I’m am 100% convinced of their authenticity and they will included in his catalogue.’

Works by Mondrian, often in private ownership, turn up every so often, not only portraits but other figurative work as well. Some 30 ‘new’ works have been added since a catalogue containing 1,400 recognised paintings by Mondrian was put together in 1998.

Mondrian painted some 93 portraits in all. Recently other examples of other figurative art by Mondrian were bought by the Mondrian house in Amersfoort.

Mondrian left the Netherlands and figurative art behind in 1915. In Paris his work gradually became more abstract culminating in the characteristic black lined geometric shapes in primary colours that made him famous.

The newly discovered portraits remain in private ownership and cannot be seen by the public.