Fifteen words, dashed off in haste five years ago, have come to play a potentially pivotal role in the General Election campaign.

As he quit his office in the Treasury in May 2010 after 11 months as chief secretary, Labour’s Liam Byrne scribbled a note to leave on the desk for his successor. It read: “Dear Chief Secretary, I’m afraid there is no money. Kind regards – and good luck! Liam.”

Liam Byrne
Liam Byrne: Former Chief Secretary to the Treasury and Labour’s official banter-general (Kirsty Wigglesworth/AP)

Intended as a private moment of “dark humour” between political rivals fated to grapple with the fallout from the worst financial crash of modern times, the note was to become a party political weapon in the hands of the Conservatives.

Since its existence was first revealed by short-lived Liberal Democrat chief secretary David Laws, the Byrne note has regularly been wielded by coalition ministers seeking to drive home their argument that Labour spent too much.

David Cameron has taken to carrying it around with him on the campaign trail, producing it from his pocket in a TV debate on yesterday’s Question Time election special and again during a visit to Asda’s HQ in Leeds.

David Cameron
And here it is last week, being brandished in the quiet Somerset village of Norton-sub-Hamdon (Chris Radburn/PA)

Ed Miliband was put on the spot by a businesswoman in the Question Time audience, who demanded to know how Ed Balls could be trusted with the economy when he described the note as a “joke”.

Byrne has said he will “probably” regret writing the note for the rest of his life, but had assumed it would remain private.

He said there was a long tradition, dating back as far as the 1930s, of ministers leaving humorous messages to their successors.

The only previous example of a similar note being made public came in 1964, when Conservative chancellor of the Exchequer Reginald Maudling left a message in his Whitehall office after being forced out by election defeat, telling his Labour replacement James Callaghan: “Good luck, old cock…. Sorry to leave it in such a mess.”

Reginald Maudling
Reginald Maudling: Loved writing to his “old cock” (PA)

There have been rumours in Westminster that Byrne expected his successor to be Tory shadow chief secretary Philip Hammond, whose serious-minded and unflashy approach to politics might have led him to assume it would be treated with discretion.

But the Lib Dems demanded the Treasury’s number two job as a condition of coalition, Hammond was sent to the Department for Transport, and David Laws waited only five days after his appointment to announce the existence of the note at a press conference.

The Lib Dem minister – forced from office less than a fortnight later over expenses – told reporters he had assumed Byrne was leaving him “advice… on how I conduct myself over the months ahead”, but instead found that it was “honest but slightly less helpful advice than I had been expecting”.

David Laws
David Laws: The maverick chief secretary Byrne hadn’t banked on… (Andrew Winning/AP)

Apologising a matter of days later (during which time Laws had been replaced by current chief secretary Danny Alexander) Byrne admitted it had been “foolish” to pen the letter.

“The truth is there is dark humour between politicians at Westminster even when you are knocking ‘things’ out of each other,” the Birmingham Hodge Hill MP said. “I broke the golden rule, which is not to write down anything you’re not happy seeing in public.

“I am sorry that it made me look flippant about deficit reduction when anybody who’s worked with me at the Treasury knows I take deficit reduction deadly seriously.”

Liam Byrne
Byrne’s “deadly serious” face (Lewis Whyld/PA)

Looking back on the incident three years later, Byrne said he was “disappointed that some very old conventions had been cast aside for political advantage”.

But, even then, he could have had no idea how long-lasting the damage to Labour’s reputation would prove.

Ed Balls was confronted over it in a radio interview last month, and insisted it was “a jokey note” which was “supposed to be a piece of humour”, adding that “of course the money hadn’t run out”.

Ed Balls
Ed Balls: It was a “jokey note” (Jon Super/AP)

His comment was denounced by David Cameron as “frankly the most appalling thing I have heard in this election campaign so far”, and the Prime Minister has not stopped reminding voters of it since.

Whether the note will play a decisive role in voters’ choices on May 7 will probably never be known, but Tories certainly regard it as a potent weapon and hardly a day can go by when Liam Byrne – and Ed Miliband – wish that it was never written.