Tony Blair plotted to drag Britain into the euro in a secret push that was opposed by his own ministers, official papers have revealed.

The former Prime Minister was so determined to ditch the pound that his advisors drew up plans for a referendum in 2004, despite fierce resistance from then-chancellor, Gordon Brown.

Documents released by the National Archives expose the extraordinary lengths Mr Blair went to in his obsession with the European project.

They reveal that just four days after Mr Brown told the House of Commons that Britain was not ready to join the euro, a senior aide was urging the Prime Minister to push ahead regardless.

Arnab Banerji, Mr Blair’s senior economics advisor, wrote a memo on June 13 2003, claiming the benefits of joining would be so huge they could pay for a “whole second NHS” within 30 years.

In the extraordinary document, titled “The Economic Case for the Euro”, Mr Banerji argued that signing up to the single currency would generate a benefit of some £3billion a year for the UK economy.

He wrote: “[The benefit] compounds over 30 years to an annual increase in output equal to between 5% and 9% of GDP.”

Mr Banerji added that the NHS at the time cost about 7% of GDP. The choice for the nation was “quite stark”, he wrote. “In 30 years’ time, we can enjoy benefits equivalent to a whole second NHS or have poorer services and higher taxes than would otherwise have been the case.”

The papers, reported in the Financial Times, show that many of Mr Blair’s inner circle of aides accepted the argument and were determined to press ahead with euro membership.

They even mapped out the details of a potential referendum, and suggested who would be able to vote if it went ahead. Papers even revealed they had settled on the question: “Should the United Kingdom adopt the Euro as its currency?”

Aides had also drafted a more positive version of the statement that Mr Brown delivered to the Commons on June 9, 2003. In the version penned by unelected aides, Mr Brown would have stressed the benefits of joining the euro “more powerfully” the Financial Times reported, and would have even opened the door to a referendum on membership in 2004.

But the documents lay bare the bitter rift between Mr Blair and Mr Brown over the issue. They include minutes of meetings in the heart of government, which had been intended to ensure the two men agreed on messaging, after reporting of their differences.

One official, the late Jeremy Heywood, was reported to note that a ministerial group working on sacking off the pound in favour of the euro would not be called a “Cabinet committee”, despite being made up of Cabinet ministers, as well as actually holding its meetings in the Cabinet room itself.

Instead it would be known as an “informal ministerial group” to “meet GB’s concerns”, Mr Heywood wrote. GB is understood to reference Gordon Brown. The tension between the two most powerful men in government continued throughout Mr Blair’s time at No 10. Mr Brown then killed off any hope of euro membership as soon as he became Prime Minister in 2007.