The Church of England will not adopt a set of cast-iron criteria when deciding whether to baptise asylum seekers as it was impossible to see into someone's heart, a bishop said on Tuesday amid a row over fake conversions to Christianity.

The centuries-old church launched a review into its advice to clergy last month to address criticism, some from within the Christian community, that it was not doing enough to stop fake conversions to Christianity to help with asylum claims.

Concerns came to a head after media reports that a man from Afghanistan, who was accused of being involved in a chemical attack in London, was initially denied asylum in Britain but had his case overturned after becoming a Christian.

Rev. Guli Francis-Dehqani told a parliamentary committee on Tuesday that the Church - central to the Anglican Communion of 85 million believers worldwide - may tighten up its language. But she added:

"How do you assess something that is in somebody's heart? It is very, very difficult to do that... Some of the Home Office (interior ministry) approach and tests have been exactly that, trying to see what people's knowledge might be about the Bible," Francis-Dehqani said.

"I don't believe there will ever be a set of cast-iron criteria," she said when asked whether there would be a fundamental change in the current guidance, which states a need for clergy to be "wise as serpents and innocent as doves" when ministering to asylum seekers.

Migration minister Tom Pursglove told the same hearing that there was no specific evidence of churches facilitating high levels of false asylum claims, but he said some elements of the church's guidance was "rather political".

He added that no one should do anything that would support "the business model of criminal gangs who bring people across via those unsafe journeys".