Campaign group Fathers4Justice said one of its members was responsible for damage to a portrait of the Queen, in an apparent "Father's Day protest".
A spokesman said it was not an official protest.
Painted by Australian-born Mr Heimans, the piece measures 9ft by 11ft and depicts the monarch in the Sacrarium of Westminster Abbey, also known as the Coronation Theatre.
The painting is entitled The Coronation Theatre, Westminster Abbey: A Portrait of Her Majesty Queen Elizabeth II.
It was shown publicly for the first time in September by the Australian governor-general Quentin Bryce at the National Portrait Gallery in Canberra.
It was moved to London and put on display in Westminster Abbey last month.
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A cloudy and damp start for many southeastern areas, but turning brighter through the afternoon.