The chair of the commons Justice Select Committee has written to the prime minister expressing ‘great concern’ over plans to change the Children Act to promote shared parenting.

In a robust letter Sir Alan Beith sets out the cross-party committee’s opposition to the government’s proposal to introduce a legal presumption of ‘shared parenting’ where relationships break down, which he argues will undermine the present responsibility to safeguard the rights of the child.

Beith said he was writing to David Cameron because: ‘It is widely believed that you have taken a personal interest in it and have significantly influenced the direction of policy making.’

He described the proposed change as ‘seriously flawed’ and based on the need to ‘remove a perception which has no foundation in fact’ as there was no evidence to suggest that judges are not starting from a position in favour of contact being maintained with both parents.

Beith told Cameron that to promote shared parenting through legislation ‘undermines the paramount principle of the welfare of the child’ and would result in ‘extensive’ litigation between parties to reconcile the two principles.

He said it would be wrong to imply by a change in the law that parents have rights over children rather than responsibilities for children, and suggested that legislation promoting shared parenting would be equated in the minds of warring parents as a right to equality of time.

Evidence from Australia, which introduced a similar change, shows that cases where a child’s or parent’s safety was at risk were not effectively filtered out of the shared parenting scheme by the courts.

The final report of the Family Justice Review recommended that no legislative statement promoting shared parenting should be introduced, stating that ‘the core principle of the paramountcy of the welfare of the child is sufficient and that to insert any additional statements brings with it unnecessary risk for little gain’.

Beith slammed the government for failing to engage with the criticisms of shared parenting or paying sufficient attention to the lessons learned from the Australian experience.

He wrote: ‘We remain extremely concerned that clearly expressed and well-researched conclusions of our committee and the Family Justice Review are being ignored. We would ask you to respond to our concerns, and explain why the government considers such reforms to be of benefit to families, and in particular, children.’