Illegal immigrants form an increasing share of Roman Catholic congregations in London, a new study has found.
Researchers spoke to parishioners in Westminster
It showed that more than 75% of parishioners in at least three parishes were "irregularly" in the UK.
The Von Hugel Institute's report said attendance has been boosted in recent years by migrants from the EU, the Far East, Africa and South America.
Westminster, Southwark and Brentwood dioceses commissioned the study to improve assistance for new migrants.
The report, entitled The Ground of Justice, stated the Cardinal, archbishops and bishops must appreciate the extent to which they are ministering to people who live in fear, and at grave risk, because of their "irregular" status.
It continued: "This means that there are now at least tens of clergy knowingly and unknowingly ministering to... parishioners who are 'irregular' or 'undocumented' in terms of their presence in the UK."
The authors spoke to 1,000 migrants attending Mass in London in the dioceses of Westminster, Southwark and Brentwood as well as migrant focus groups.
Researchers said some parishioners would not answer questions as they were fearful of the consequences if they were identified.
Several priests also refused to take part in the survey because of the duty of care and confidentiality they felt they owed their parishioners.
A statement issued by the Cardinal, the Archbishop of Southwark the Most Rev Kevin McDonald and Bishop of Brentwood the Rt Rev Thomas McMahon responded to the findings.
They said: "Migrants are very much the present reality of the Catholic Church in London and one of several sources of hope for the Catholic Church of the future too.
"In our view they are also a source of hope for the future of the nation."