Pope Benedict XVI tonight attacked the "increasing marginalisation" of Christianity, arguing that religion should be recognised for its "vital" contribution to the nation.
The Pontiff told MPs, peers, and religious leaders in Westminster Hall that there were "worrying signs" of a failure to appreciate the rights of believers to freedom of conscience and the "legitimate" role of religion in public life.
In his speech, the Pope said he was voicing his concern at the growing marginalisation of religion - particularly of Christianity - even in nations which place a great emphasis on tolerance.
"There are those who would advocate that the voice of religion be silenced, or least relegated to the purely private sphere," he said.
"There are those who argue that the public celebration of festivals such as Christmas should be discouraged, in the questionable belief that it might somehow offend those of other religions or none.
"And there are those who argue - paradoxically, with the intention of eliminating discrimination - that Christians in public roles should be required at times to act against their conscience."
The Pope's remarks are likely to be interpreted as aimed in part at the failure of the Catholic adoption agencies to retain the right to refuse gay couples as potential adoptive parents.
The Church has also come under fire over the right to run faith schools and whether it should be allowed to bar sexually active gay people from key appointments.
In his speech, the Pope highlighted Catholic martyr St Thomas More, who was tried in Westminster Hall and sentenced to death in 1535.
The Pope paid tribute to the role of Parliament and its influence in developing democratic Government across the world.
He said Catholic social teaching had "much in common" with the approach of Britain's democracy with its "strong sense" of the individual's rights and duties and of the equality of all citizens before the law.
Former prime ministers Tony Blair, Gordon Brown, Sir John Major and Baroness Thatcher were all present in Westminster Hall to hear the speech.
The Pope also highlighted areas of common concern shared by the Holy See and the Government.
There had been collaboration on debt relief, fair trade and financing for development through initiatives such as the International Finance Facility (IFF), he said.
The present Government has committed the UK to devoting 0.7% of national income to development aid by 2013, he said.
In recent years, he said, it had been encouraging to witness the positive signs of a worldwide growth in solidarity towards the poor.
But he said turning this into effective action called for "fresh thinking" to improve conditions in areas such as food production, clean water, job creation, education and support to families.
"Where human lives are concerned, time is always short - yet the world has witnessed the vast resources that governments can draw upon to rescue financial institutions deemed 'too big to fail'," he said.
"Surely the integral human development of the world's peoples is no less important - here is an enterprise, worthy of the world's attention, that is truly 'too big to fail'."
Andrew Copson, chief executive of the British Humanist Association, said: "The Pope's statements concerning the alleged 'increasing marginalisation of religion' were a parody of the real situation in the UK, where politicians increasingly move to expand state-funded religious schools, contract public services out to religious organisations and act in other ways that privilege religious beliefs and organisations in such a disproportionate and discriminatory manner."