Scottish nationalists are calling for the prime minister to repeal legislation that discriminates against Roman Catholics.
The Act of Settlement states that Catholics cannot become king or queen of England and bars members of the royal family who marry Catholics from taking the throne.
Alex Salmond, leader of the Scottish National Party (SNP), challenged Tony Blair to announce a timetable for the repeal of the act at prime minister's question time on Wednesday.
"It represents clear institutional discrimination against millions of our fellow citizens," he told the House of Commons.
"Wouldn't a government set on a course of repeal show leadership, authority and direction?"
Mr Blair, who is himself married to a Catholic, replied: "No, I can't give him that assurance."
The SNP leader said the prime minister's "petulant response" sent out the wrong signal.
"This is an issue of cross-party and cross-faith concern. Tony's Blair's blunt dismissal of repeal is unhelpful. It sends the wrong signal and undermines efforts in Scotland to tackle discrimination," Mr Salmond said.
He added that an independent Scotland - something Mr Blair said would be "an absolute and total disaster" for its people - would act to "remove this discrimination".
The act states that any person who "should be reconciled to, or shall hold communion with the see or Church of Rome, or should profess the popish religion, or marry a papist, should be excluded, and are by that act made for ever incapable to inherit, profess, or enjoy the crown and government of this realm".
In practise, the act means that Prince Michael of Kent and the Earl of St Andrews, the eldest son of the Duke of Kent, have lost their right of succession by marrying Catholics.