The NHS has told midwives about the "benefits" of cousin marriage despite the rise in birth defects.
New guidance explains that concerns about the risks of congenital diseases are "exaggerated" and "unwarranted" as "85 to 90 per cent of cousin couples do not have affected children".
The NHS acknowledged that there are some "risks to child health associated with close relative marriage", but the guidance says these should "be balanced against the potential benefits... from this marriage practice".
The document says marrying a relative, particularly common in the Pakistani community, can provide "economic benefits" as well as "emotional and social connections".
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Staff should not "stigmatise" predominantly South Asian or Muslim patients who have a baby with their cousin, describing the move as "perfectly normal".
Richard Holden is one of many critics of the advice, with the Tory MP campaigning to ban cousin marriage.
He said: "There are no benefits to marriage between first cousins, only massive downsides for health, welfare, individual rights and the cohesiveness of our society."
Patrick Nash, Oxford professor and director of the Pharos Foundation research institute, said: "This is on a par with recommending alcohol and smoking during pregnancy for their calming effects, while brushing over the absolutely horrendous consequences for mother and child.
The NHS told staff not to 'stigmatise' those who have cousin marriages
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"There is no justification or excuse for this at all.
"Shame on the authors and shame on the Government for refusing to ban this indefensible cultural practice."
The guidance was created as part of NHS England's Maternity Transformation Programme, which aims to halve the number of stillbirths, neonatal and maternal deaths and brain injuries by 2030.
It tells NHS staff that "discouraging cousin marriage is inappropriate" and would be "alienating and ineffective".
Staff have been told cousin marriages are 'perfectly normal'
| GETTYThe document continues: "The increased risk of genetic conditions among the offspring of close relative couples has often been exaggerated... leading to individuals feeling shamed and blamed.
"[Some] 85 to 90 per cent of cousin couples do not have affected children. Pakistani women in cousin marriages have been found to compare favourably to those in non-relative marriages."
The guide says there has been an "unwarranted, narrow focus on close relative marriage".
Professor of Economic Psychology at the London School of Economics, Michael Muthukrishna, said: 'When marriage is restricted to family members, communities become more isolated, limiting social integration.
NHS staff have been told that 'discouraging cousin marriage is inappropriate'
| GETTY"This isolation is what has allowed for over-representation of radicalisation and grooming gangs.
"Normalising cousin marriage doesn't help mothers nor babies affected by the well-documented health risks of repeated inbreeding."
Nearly 80 per cent of the British public, according to recent polling by YouGov, say they support a ban on cousin marriage.
Nine per cent in the poll believe cousin marriage should be legal.
An NHS spokesman said: "The NHS absolutely recognises the genetic risks of consanguineous relationships, and where people consider entering into them we offer referral to genetics services so individuals understand the risks and can make informed decisions."
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