Former prime minister Gordon Brown has warned that implementing proposals to devolve income tax to the Scottish Parliament in full and then depriving Scottish MPs of voting on the Budget would be "absolutely lethal to the constitution".

The Labour MP for Kirkcaldy and Cowdenbeath said: "You cannot have one United Kingdom if you have two separate classes of MPs."

Mr Brown said the "fundamental question" affecting the British constitution was not the West Lothian question, which he argued was a symptom of a more fundamental problem about the fair distribution of power.

Speaking during a Commons general debate on devolution following the Scotland referendum, he said: "The fundamental question in the British constitution is that when you have England 84% of the Union, Scotland 8% of the Union, Wales 5% of the Union, Northern Ireland 3% of the Union ... you have the reality that at any point the votes of England could outvote Scotland, Wales and Northern Ireland individually or collectively.

"The real fundamental issue, the real issue is a fair distribution of power, that respects not only majority rule - and I am sensitive to the needs of England and to English voters - but also respects the rights of the minorities so that we have stability and we have harmony in the British constitution."

He said every generation had had to come to terms with "how you get that balance right between the majority rule and the needs of the minorities that are part of the UK".

Mr Brown criticised the Prime Minister's move to raise the issue of English votes on English laws on the morning after the Scottish independence referendum.

He said: "Without telling people beforehand, a matter that was absolutely material to the vote that people were casting in the Scottish referendum, a new plan was imposed upon Scotland.

"It was a vow that had been written on the Tuesday, that was now being rewritten on the Friday morning, because while the proposed change was, he said, in the English constitution, the actual practical effect of it was in Scottish constitutional affairs to restrict the voting rights of Scottish MPs in this House of Commons, on an issue as he said on that morning as fundamental as taxation.

"Clearly that was a change in Scotland's status in the UK, clearly it was highly material to the vote that people had just had and should not the people of Scotland have been told prior to the referendum, which was on Scotland's status in the UK, that the downgrading of Scottish representation in Westminster was one of the proposals that he now wishes to make to the people of the country."

The former chancellor branded the plans a "lethal cocktail", adding: "The Conservative Party wants to devolve 100% of income tax to the Scottish Parliament ... and then immediately end the right of Scottish MPs to vote on income tax on a matter as substantial as the Budget in this Parliament of the UK."

He went on: "It is the combination of the two proposals to devolve 100% of income tax and then to remove the right of Scottish MPs to vote on the matter in Westminster that is absolutely lethal to the constitution."

The impact of the plans, he warned, would lead to Scottish representatives being able to vote on some of the business of Westminster but not all of it. They would not be able to vote on some Budget decisions on income tax and "would undoubtedly become second class citizens at Westminster".

Mr Brown argued that implementing Conservative proposals to devolve income tax to the Scottish Parliament in full and then deprive Scottish MPs of voting on the Budget would not work.

He said: "If we can avoid making the kind of mistakes that the Leader of the House is now making, if we can rise above narrow partisan interests and put country before party, if we can remain statesmanlike in seeking unity as the siren voices across there try to reap discord, then Britain still can be the Great Britain that we want it to be."