Britain's leading midwife, Cathy Warwick has accused prime minister David Cameron and the health secretary, Andrew Lansley, of risking the safety of mothers and babies by backtracking on pledges to hire more NHS midwives.

Maternity units could be forced to offer inadequate and potentially unsafe care because hospitals cannot cope with growing demand caused by the rising birthrate and an increasing number of complex pregnancies, warned Warwick, the general secretary of the Royal College of Midwives. "I fear for the future of maternity services, that the quality of care will fall and that safety could be compromised," she said.

"I fear that midwives who are toiling away will become even more disillusioned. Most of all I fear that women and their babies will be ill-served by maternity services," she said at the union's annual conference in Manchester today.

She added: "I am very concerned that the needs of pregnant women are greater than ever before, yet we are still acutely short of midwives."

Warwick said the NHS in England needs 3,500 more midwives to ensure every woman and baby receives proper care. She highlighted an article by Cameron in the Sun in January, in which he said: "We are going to make our midwives' lives a lot easier. They are crucial to making a mum's experience of birth as good as it can possibly be, but today they are overworked and demoralised. So we will increase the number of midwives by 3,000."

She said: "We are extremely disappointed ... Both coalition parties supported a commitment to more midwives; now they have apparently changed their minds, and yet the economic situation was well-known before the election."

She had encountered "deafening silence" from the government when she had asked if it intended to honour the pledge, and had had no response to a letter she wrote in September to the prime minister, she added.

The Department of Health rejected Warwick's claims, saying that Lansley had "made it clear in a recent meeting with the RCM that the government will continue to train midwives at current rates and we are considering ways of helping improve midwife recruitment and retention".

A Conservative party spokesman said: "The commitment to 3,000 midwives made in opposition was dependent on the birthrate increasing as it has done in the recent past. It was not in the coalition agreement because predictions now suggest the birthrate will be stable over the next few years."

At prime minister's questions, Cameron defended himself but did not discuss actual numbers. "We do want to see an increase in midwives and, unlike the party opposite, we're actually funding the health service in a way that makes that possible," he said.

In an RCM survey earlier this week, many heads of midwifery in England reported that they had had, or expected to have, cuts in their staffing levels or budgets.

Warwick also lambasted Lansley for first proposing that maternity services would be commissioned by the planned new NHS commissioning board together with maternity networks, but then deciding to hand responsibility to the new consortia of GPs. "This is short-sighted … a big mistake,"