A St John's Wood museum has handed over Mark Gertler's portrait of diplomat Sir Sydney Waterlow to the National Portrait Gallery so it can be more widely seen.

The tiny Ben Uri Gallery in Boundary Road said the inter-museum transfer was part of their commitment to release hidden works from long-term storage for the public benefit.

Waterlow (1878-1944) was the grandson of the Highgate philanthropist who donated Waterlow Park to the public as a "garden for the gardenless". He was also part of the famous Bloomsbury Group, unsuccessfully proposing to Virginia Woolf, and befriending Gertler, who had a passionate, mostly unrequited, relationship with fellow artist Dora Carrington.

Rosie Broadley, NPG Head of Collections Displays said: “We are so grateful to Ben Uri Gallery for generously transferring this important work. It’s a welcome addition to the Gallery’s Collection, which includes a number of key portraits depicting the sitter’s Bloomsbury contemporaries.”

Ben Uri Director Sarah MacDougall, said: “We are delighted that this important work has found the right home in such a distinguished national collection, alongside other Bloomsbury portraits by Gertler and others”.

First exhibited in February 1922, the portrait captures Waterlow - known affectionately as "Monarch" - mid-career. He was a member of Gertler's all-male group of mostly writers and intellectuals - known as "the Thursdays" - who met weekly at his Hampstead home throughout the 20s and 30s.

Born in Spitalfields to Austrian-Jewish parents, Gertler was raised in the East End, but his talent was spotted by artist William Rothenstein who encouraged him to attend the Slade School of Fine Art. There he met Carrington and went on to socialise with members of the Bloomsbury Group, including artists Vanessa Bell, Duncan Grant and Roger Fry. But he fell out with Lytton Strachey when Carrington left him to live with the writer.

A conscientious objector and pacifist, Gertler had moved to Well Mount Studios, Hampstead in 1915 and the following year painted his famous anti-war work Merry-go-Round, drawn from a fairground ride on Hampstead Heath. He lived at other Hampstead addresses including the Vale of Health and Haverstock Hill and after his disappointment with Carrington, went on to marry and have a son.

But in 1939, poverty stricken, suffering from tuberculosis and in a failing marriage, he took his own life in his studio at 5 Grove Terrace, Highgate Road.