The Duchess of Cambridge has sympathised with the mental health plight faced by many mothers, saying how generally they were supposed to be "super happy" but one in four were not during a hospital visit in Beckenham.

Kate's comment came as she visited the NHS mother and baby unit in London that treats women with serious psychological issues including those threatening suicide at Bethlem Royal Hospital.

Speaking during her visit, the pregnant mum-of-two duchess told a group of psychiatrists, midwives and health visitors specialising in treating mums with mental health issues, that "particularly as a mother, there's an expectation to be super happy all the time and one in four of us aren't".

She was quoting research data from a study by Professor Louise Howard from the South London and Maudsley NHS Foundation Trust which runs the mother and baby unit.

During her visit she joined a mother in a sensory room with her young baby - a facility which helps to relax women and their children - and chatted privately to a group of mums with their infants, some only a few weeks old.

Dr Gertrude Seneviratne, lead of the perinatal mental health services and chair of the Royal College of Psychiatrists’ perinatal faculty, said: “The arrival of a baby should be the happiest time in a woman’s life, but for some women this is not the case - our research shows that one in four women experience mental health problems during pregnancy or shortly after the baby is born.

“Mothers often arrive here in much mental distress, feeling that they can’t do things for themselves, but by keeping mother and baby together and providing a range of specialist treatment and therapy, women do recover and can bond with their child.

“Today is a very special occasion for us. It means a great deal to staff and patients that the duchess was able to spend some time meeting them – especially as she is a mother herself.”