Two out of five mothers feel they are discharged from hospital too soon after giving birth, according to a survey that raises fresh concern about NHS postnatal care.

Research undertaken by the Royal College of Midwives (RCM) and the Netmums website found that 40% of new mothers felt they were sent home before they were ready.

Of those, 9% said they felt "rushed out", while the other 31% said they were "keen to get home, but also felt unsupported and not quite ready to leave".

However, 29% of mothers said staff "made me feel I was happy to stay until I was ready" and the other 31% had to stay longer than they would have wished because they or their baby was not yet well enough to go home.

The findings, from a survey of 486 mothers undertaken last year, raise concern that some hospitals may be breaching guidance from the National Institute of Health and Care Excellence (Nice), the government's health advisers. It says that "length of stay in a maternity unit should be discussed between the individual woman and her healthcare professional, taking into account the health and wellbeing of the woman and her baby and the level of support available following discharge".

But, the RCM says, "this is clearly not happening". It said the significant minority of women leaving hospital before they felt ready was "a real concern". An England-wide shortage of midwives is also to blame, it added.

Mothers' average length of stay in hospital has been falling for about 15 years, and most leave within six hours of their delivery.

One mother who took part in the survey said: "I had great care with my first baby, but with the second I was rushed home within a couple of hours of giving birth, distressed and in pain, [and] probably still in shock, which contributed to my postnatal depression."

The survey findings are contained in a report published on Friday by the RCM, which warns that some mothers and their babies are receiving inadequate care in the days and weeks after the birth, both in hospital and when they get home. In it, one of more than 2,000 midwives who undertook a separate survey recounted: "The management told me I had to discharge this woman who was struggling with breastfeeding as we needed the bed. There is little support for the women due to our early discharge policy."

The NCT, a charity which supports new parents, said it was unacceptable for mothers to be sent home before they felt comfortable about going.

"Sometimes new mums want to get home quickly but it is unacceptable if they are discharged early because of staff shortages or lack of beds. Giving birth can be a stressful time and the last thing new parents need is to be sent home before they feel ready," said Belinda Phipps, its chief executive.

"It is also deeply frustrating that Nice guidelines on postnatal care are still not being implemented. We need to see a rise in the number of midwives and better continuity of care so women have access to a midwife they know and trust – which leads to better outcomes for parents and babies."

Maternity units across the country are under severe pressure because of a baby boom.