The Archbishop of Canterbury, Dr Rowan Williams, has travelled to Rome to celebrate the 100th anniversary of the birth of the late Dutch Cardinal Johannes Willebrands, a campaigner for the unification of Christianity. But when Dr Williams holds a private meeting with Pope Benedict XVI today, the mood is likely to be one of animosity, rather than ecumenicism.

Last month saw one of the most divisive acts by the Catholic Church in decades. The Pope unveiled an "apostolic constitution" which would allow practising Anglicans to join the Catholic Church. Under the new arrangement, Rome would even admit married Church of England clergy and allow entire congregations to continue using their traditional liturgy and prayer book. The Archbishop has been quietly criticising the Catholic move in recent days. But behind closed doors he ought to be more direct.

The Vatican claims that the invitation was entirely a response to requests from disgruntled Anglicans to be allowed to join the Catholic communion. But this affair looks more like an attempt by Vatican conservatives to exploit tensions within the Anglican Church over women bishops and gay priests for their own gain.

Tellingly, the Vatican's Council for Christian Unity played no part in devising the new arrangements. And Dr Williams was not informed of Rome's intentions until virtually the last minute. This whole business has served to undermine the Archbishop's authority in the midst of a bitter struggle for the very future of Anglicanism. Dr Williams should tell the Pontiff that there is nothing ecumenical about trying to poach congregations. And he should make clear that talks on a closer union of Anglicanism and Catholicism cannot continue while Rome engages in such unilateral power grabs.

As regards his own flock, the Archbishop should continue to make efforts to hold the centre, seeking out innovative ways to allow freedom of conscience for all. Compromise should be sought (although not at the expense of alienating women and homosexuals). Unity is a noble mission, even if Dr Williams will apparently have to pursue it with hindrance, rather than help, from Rome.