A row has broken out between employers and the union Amicus after it was accused of advising people how to take "sickies" to watch the World Cup.
Amicus warns absence without permission can lead to trouble
On its website, the union says it is "difficult to prove someone is not really sick" and offers a line of defence to workers who are caught out.
One small business organisation says it is "appalled" and that Amicus should withdraw its World Cup Fever article.
The union insists it does not condone dishonest sick leave.
But Stephen Alambritis of the Federation of Small Business says that, with absence costing the UK economy £13bn a year, it does not need a major trade union offering guidance on the best way to throw a "sickie".
He told the BBC that publishing such advice was "grossly unfair on all staff" and he urged Amicus to withdraw its advice.
The Amicus website asks: "So you want to watch the World Cup, but you are meant to be at work when it's on.
"Can you play away or is the risk of permanent relegation from your job too high?"
If union members fail to book time off to watch World Cup matches, they could try to persuade bosses that watching a game together at work could be "a great team building" exercise, the website says.
It goes on to say that "it is quite difficult to prove that someone is not really sick if they have one day off".
"Taking time off work without permission can lead to dismissal for gross misconduct," it adds.
"Also, lying to your employer about your reason for absence might amount to gross misconduct too."
But the website advises that if the absence procedure of a company "does not make this clear", members "can argue that it is simply a form of misconduct which should be viewed in the light of your work record".
It advises that a union representative "should be able to help you with your arguments and interpretation of procedures."
The head of legal affairs at Amicus, Georgina Hirsh, denied that the union was encouraging dishonest sick leave.
She told BBC Radio 4's Today programme: "On balance the article is far from encouraging people to take sickies, and in fact we advise people that it's a big risk for them to do so."
She said unions existed to help advise people about issues at work. "I'm afraid the reality is - lots of people do take sickies whether it's for the World Cup or not," she said.
And at least one employer told the BBC that he planned his staff rota around the World Cup matches.
Mark Eagle of Reigate told the BBC News website:
"I have 20 staff from 6 World Cup nations. I have made the decision when doing staff rotas to look at the fixtures list when compiling them to ensure I don't get "sickies" - as otherwise the business would not operate."
Would you take sick leave to watch the World Cup? What excuses have you used?