Changes to the laws of royal succession will face their first Commons test on Tuesday.
The legislation will end discrimination against female royals so that men will no longer take precedence over women in the order of succession - meaning the Duke and Duchess of Cambridge's first child can become monarch even if it is a girl who later has younger brothers.
The proposals will also end the bar on royal heirs marrying Catholics if they want to keep their entitlement to the throne.
The Government's handling of the changes has been criticised by constitutional experts and the plans have reportedly led the Prince of Wales to raise his concerns with senior officials.
The legislation is being rushed through the Commons with just two days set aside for debate, but Deputy Prime Minister Nick Clegg insisted it was time to bring the "arcane laws" up to date.
Speaking ahead of the Succession to the Crown Bill's second reading debate, Mr Clegg said: "The reforms couldn't be more timely, given the fantastic news that the Duke and Duchess of Cambridge are expecting a baby.
"The other Commonwealth countries where Her Majesty The Queen is head of state have just given us the green light to change the law, and we are wasting no time. At the moment, if the first child of the Duke and Duchess of Cambridge is a girl, any younger brothers she has will overtake her in line to the throne. We're modernising these out-of-date rules so that men and women in line to the Throne have equal rights.
"The current law also says that our monarch can't be married to a Catholic. This legal ban doesn't apply to any other faith - not Muslims, Jews, Hindus, nor to atheists. Just Roman Catholics. The reasons for this go back 300 years, to the days when Britain was worried about the threat from its Catholic neighbours, such as Louis XIV of France.
"Times have changed, along with our attitudes towards each other. It is time for us to bring these arcane laws up to date."
The Lords Constitution Committee has criticised the speed with which the Commons was being asked to pass the Bill and warned that proper debate was needed to tease out potential problems that have been "overlooked or hidden".