The problem, according to the Swiss company, is that the old slogan has become so well-known that it no longer motivates people to buy more Kit Kat bars - a major cause for concern in a £3.7 billion market (the biggest in Europe) but where growth is hard to come by and competition is fierce.
The company said that the new slogan reflected the changes in British society over the last half century. Britons no longer need to be urged to take time out from working for indulging in chocolate and other little luxuries, according to Nestlé - the key now is to ensure that they profit from their breaks by choosing the right product.
But according to a report in the Financial Times, the company is not giving up entirely on the old slogan - it could be reinstated in three or four years when Nestlé Rowntree has "re-established the relevance of the brand".
Kit Kat has been Rowntree's best-selling product since 1937, and the first TV advert for the brand, when the 'Have a break…' slogan was used for the first time, appeared in 1957.
Competition, in particular from Cadbury's Dairy Milk brand, has become increasingly intense over the last 10 years or so, prompting Nestlé Rowntree to look at new ways of rekindling interest in the brand. These have included limited-edition flavour variants (such as orange or mint) and spin-offs such as Kit Kat Kubes and the hugely popular Kit Kat Chunky which appeared in 1999 and became Nestlé's most successful launch of the decade.
While these launches helped keep Kit Kat at the top of the countline market in recent years, Cadbury's revamp of Dairy Milk - driven in particular by the launch of a large number of flavour variants - finally pushed it into top spot last year, according to market analysts Datamonitor. Cadbury's latest move, as we reported last week, was the launch of a Kit Kat clone - Dairy Milk Wafer - designed to consolidate its position as the new number one and keep up the pressure on the erstwhile market leader.
But is changing the strapline - especially one which is so well-known - really likely to be effective in helping Kit Kat regain its crown?
"Mars changed its classic 'Work, rest and play' slogan in 2002 and saw a rise in sales the following year," Datamonitor said. "So, as Kit Kat struggles to regain the countlines crown from Cadbury, this shift in emphasis could yet be a winner."
The Mars situation was very similar to that facing Kit Kat, according to Datamonitor. "In the face of falling sales, the company abandoned the 'A Mars a day helps you work, rest and play' slogan that it had been using for decades and replaced it with 'A pleasure you can't measure'. The move was greeted with a degree of scepticism from industry sources, but over the following year, sales of Mars bars grew by 20 per cent.
According to the analysts, Nestlé Rowntree's decision to target workplace snacking, while still retaining its former association with relaxation, could be a shrewd one. "Workplace consumption is a regular and increasingly frequent occurrence, which presents a growing opportunity for consumer packaged goods players.
"The market for confectionery and snacks in the workplace is currently worth €140 billion in western Europe. Consumers do not just eat and drink at work out of simple necessity, but also because of the psychological need for stimulation and reward."
The pressure from Cadbury has clearly influenced Nestlé's thinking over the last 18 months or so. Nestlé had been trying to register the slogan 'Have a break' as a trademark since 1994, but had its request turned down by the High Court back in December 2002 after a guerrilla campaign from Mars, which claimed that 'Have a break' could not be registered as it had no distinctive character - unlike the already-registered 'Have a break, have a Kit Kat' strapline.
At the time, Mars was rumoured to be considering the launch of a new product called Have a Break, which the Swiss group was concerned would be confused with Kit Kat. But that product failed to materialise, as did Nestlé's appeal against the High Court ruling, as the pressure on Kit Kat began to come not from Mars or a similar-sounding product, but rather from Cadbury and its Dairy Milk brand.