The letter signed by Birmingham Archbishop Vincent Nichols was also sent to more than 2000 secondary and primary Catholic schools, the UK Daily Telegraph reports.
It follows a decision taken by all of Amnesty's 70 international groups in August to abandon the group's neutral policy on abortion as part of its Stop Violence Against Women campaign.
The group's International Committee voted to "support the decriminalisation of abortion, to ensure women have access to health care when complications arise from abortion, and to defend women's access to abortion, within reasonable gestational limits, when their health or human rights are in danger".
Amnesty International was set up in 1961 by Peter Benenson, an Oxford lawyer and convert to Catholicism. It has huge Christian support among 1.8 million members.
But its abortion policy has triggered an exodus of Catholics. Cardinal Keith O'Brien, the Archbishop of St Andrews and Edinburgh, resigned with "great sadness" after more than 40 years with the organisation, as did East Anglia Bishop Michael Evans.
Catholics ban Amnesty over abortion policy (Daily Telegraph, 17/11/07)
LINKS (not necessarily endorsed by Church Resources)
Catholic Bishops of England and Wales
Archbishop Vincent Nichols (Wikipedia)
Schools set up Amnesty pro-life alternative (CathNews, 18/11/07)