"The statement that Britain will be the last country not in the euro zone is a statement of fact.
But Chancellor Gordon Brown - who insists on a "prepare-and-decide" policy - has denied Conservative Party claims of a rift with Mr Cook.
The Tories had suggested Mr Cook dropped positive references to the euro from a parliamentary speech on Thursday, because they differed from the chancellor's Mansion House speech made later in the City.
An initial draft of Mr Cook's speech called for the country to resolve in principle to join the euro, declaring the government would not let the UK lose out by staying out.
Mr Cook said he had dropped certain passages only because he was "dipping in and out" of his text during the course of a live debate.
'Nothing dropped'
The foreign secretary told the BBC he stood by everything that was in his written text.
"I did not drop anything from my speech," he said.
"The statement that Britain will be the last country not in the euro zone is a statement of fact.
"The statement that this government will not let Britain lose out by staying out of the euro is something I have said half a dozen times and will say again because it is true."
Mr Brown told the BBC that any changes made to that speech were not requested by him.
'Mirroring the Tories'
But shadow chancellor Michael Portillo told BBC One's Breakfast News it was "clear that the cabinet is in disarray", and he warned that there would be a heavy electoral price to pay.
He said Labour's current ministerial differences over the euro mirrored those that the Conservatives experienced three years ago.
"We remember that the Conservatives paid a heavy price for that and Labour is going to pay a heavy price for all this public squabbling in the cabinet," he said.
Shadow foreign secretary Francis Maude said: "This is Gordon Brown wielding the big blue pencil.
"Robin Cook was planning to say that the government would not let Britain stay out of the euro and was very explicit about that. Gordon Brown clearly stamped on that."
Warning for Tories
However, the Tories' divide on the issue was also revived, when gormer Conservative chairman Chris Patten warned against the party's drift towards euro-scepticism.
He issued a barely-veiled attack on its decision to accept support from anti-Europe businessman Paul Sykes.
Mr Patten made clear that he considered it dangerous to allow people of Mr Sykes' opinions to dominate debate on Europe.
Pro-euro former Liberal Democrat leader Paddy Ashdown called on Tony Blair to show a firmer lead on the issue.
Mr Ashdown said: "It is dangerous for Britain, which is being left leaderless on the biggest issue ahead of us, and above all it is dangerous for Mr Blair."