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For Gordon Brown - to whom the Bill means so much - and Geoff Hoon, his chief whip, the prospect of a parliamentary defeat on its measures is unthinkable.

Yet that is the uncomfortable position in which they find themselves as a result of the implacable opposition of the Roman Catholic lobby; and it is why, eventually, the two men may grudgingly be forced to offer more concessions to those Catholic Labour MPs who are threatening a Commons revolt.

The Bill is designed to enable research that could lead to significant advancements in the treatment of killer diseases. However, its opponents believe its proposals are ethically wrong and that Catholic ministers should be allowed to exercise their consciences and vote against it.

The outline of the battle is straightforward; the underlying tensions and bitterness that could result from it are more complicated, but are nonetheless vital to understanding what is at stake for Mr Brown's Government as the affair threatens to become one of the most serious crises of his premiership.

The Bill was part of the Prime Minister's first...