Good Lord! Can that really be nodding Cook mocking his leader?

Robin Cook has broken the habit of a lifetime. In a speech yesterday he dared to speak out publicly against a policy advocated by Tony Blair. What is more, he went as far as to mock the policy supported by his leader. Mr Cook is in an unusually pugilistic mood.

The issue that has placed a senior Cabinet minister against his own leader is the reform of the House of Lords. During Prime Minister's Question Time on Wednesday, Mr Blair declared out of the blue that he was in favour of a second chamber that was fully appointed. Mr Cook supports, with some passion, a largely elected second chamber.

As Leader of the House, he sits close to Mr Blair on the front bench, and is therefore in the unfortunate position of being caught on camera listening to every word the Prime Minister delivers. Normally, Mr Cook is filmed nodding enthusiastically, even if the nods disguise the occasional private doubt. Sometimes he can be seen whispering helpfully into Mr Blair's ear as a backbencher poses an awkward question. But on Wednesday Mr Cook, who is a great racing fan, looked as if he'd just heard that a horse on which he had placed a £50,000 bet had fallen at the first hurdle. As his leader outlined his vision of an appointed second chamber, the former foreign secretary tried to appear unfazed, but he was unable to pull it off.

At a big Fabian conference in London yesterday the Leader of the House did not even try. First of all, he had a dig at the Government's original White Paper on the reform of the House of Lords, the one largely prepared by Mr Blair's old friend, the Lord Chancellor, Derry Irvine. The contorted proposals published at the time included only a 20 per cent elected element. Mr Cook observed yesterday, with a hint of sarcasm: "It would be a fair summary that the White Paper was not well received." Then he proceeded to take on Mr Blair's new proposal in a way that can only be described as mischievously daring: "We need to remind ourselves that the problem was not that the White Paper was seen to offer a dangerously high level of elected members. Press, public and Commons alike widely panned it for not providing a larger democratic element. I am therefore doubtful whether it will be possible to secure public confidence in our proposals by going in the opposite direction and reducing the elected element to zero."

Ouch! I cannot stress enough how unusual this is. Mr Cook is a funny speaker, but his wit is nearly always directed at political opponents. Privately he is capable of stirring it a bit but, compared to many of his colleagues, he has been - at least until now- an instinctive and serious-minded loyalist. Oddly, this is not his reputation in either Downing Street or the Treasury. Quite unfairly, he was blamed at No 10 for manoeuvring against the Prime Minister's position on Iraq last autumn. In reality, Mr Cook made some innocent comments to a journalist that were blown out of proportion. On the same day Clare Short, the Secretary of State for International Development, was incomparably more critical in an interview with me on GMTV. She was rewarded with a convivial private meeting with Mr Blair; Mr Cook was dismissed as a schemer – but I have not seen any evidence of this. Indeed, his allies sometimes despair that he has been too bloody loyal, while No 10 sniffs disloyalty and the Chancellor, for reasons that have never been entirely clear, cannot stand him. Mr Cook has managed to be a team player who arouses considerable suspicion in the rest of the team.

Not that the Leader of the House is being overtly disloyal now. The future of the Lords will be decided in a free vote in the Commons on Tuesday. Ministers can vote as they wish. Even so, the ministerial clash over reform is a moment of some significance, partly because of the timing. Labour MPs, well beyond the usual suspects, are deeply uneasy, wondering where Mr Blair is heading on Iraq and public service reform. Now the Prime Minister has signalled the limits of his reforming zeal for the constitution. It was already clear there was going to be no electoral reform for the Commons. Suddenly we know that, as far as Mr Blair is concerned, there will be no need for any voting system for the Lords because no one would get a vote.

I have some sympathy for that position. Britain is not overwhelmed with an abundance of elected talent. Mr Blair is not thrilled with the overall quality of the parliamentary Labour Party, from which he must draw most of his ministerial team. Some local councils are much worse off than he is in terms of their elected representatives. As Ken Livingstone observed a few years ago: "Once councils had been stripped of their powers by Margaret Thatcher you would need psychiatric help if you still wanted to go into local government." Some councillors are in need of that psychiatric help.

But the poverty of representation is not a reason for denying voters the chance to elect an upper chamber. This is partly because even a "revising" chamber still has an important legislative function, sometimes radically altering the original legislation. It would be an absurdly retrograde step to create such a powerful institution without any connection with the voters. As Mr Cook said in another withering section of his speech: "If we want the public to trust politicians then politicians must show that they trust the public to elect the right people. If we exclude the public, we should not be surprised if the public is then cynical about those who are appointed as people's peers."

The focus of the debate on Lords' reform should not be on whether an elected body is the right way forward. Other countries seem to manage with fully or partially elected second chambers. Instead, the question should be: how the heck do we get more talented people elected to an upper chamber?

It seems an age away when the Blairite agenda included closer ties with the Liberal Democrats, a more pluralist approach to governing, a referendum on electoral reform for the Commons, a more "democratic" second chamber and placing Britain at the heart of Europe. No one can accuse Mr Cook of inconsistency. In 1996, with the support of Mr Blair, he unveiled a series of constitutional reforms – agreed jointly with the Liberal Democrats – that underpinned this ambitious agenda. He still believes strongly in every item, from electoral reform a largely elected second chamber. The twist is that this puts him in opposition to Mr Blair. Although he has always been on the margins of New Labour, Mr Cook is now more Blairite than Mr Blair himself.