Copyright law is an issue that I get lots of questions about, and rightly so, because it can be a complicated business.The most recent enquiry was from a gentleman who wanted advice about using an extract from another author's work in his manuscript. This is what I told him:
If you want to include someone else's work in your own, whether a poem, song lyric, reference, photograph, map and so on, you must seek and obtain permission from the copyright owner.
The good news is that there is an exception: if you want to quote only a dozen or so words, such as a short phrase or a quotation, you can usually do so without permission, providing you reference it correctly.
Top tip: Never guesstimate the amount of words you want to use. Always check and be sure your word count is allowable.
How to seek permission? In the case of literary works, the publisher of the work is the first port of call. Many publishing houses have permissions departments which handle just this sort of thing. If they aren't able to grant permission directly, they will know who to contact.
Should you run into a brick wall, the British Copyright Council and the Copyright Licensing Agency are there to advise and point you in the right direction.
Obtaining permission can take time, so it's advisable to seek it as soon as you know (or think) you'll need it. If you don't bother or hope for the best and are found guilty of plagiarism, you could find yourself facing legal action.
Even though it's unlikely you'll do porridge as a result, you will be landed with a hefty fine, legal costs and a bad reputation. No one likes a copycat.
Best wishes, Jo
PS: see my previous post Can someone steal my idea? for tips on how to protect your own work.
Want to know more? The Writers' & Artists' Yearbook carries several articles on the tricky issue of copyright.

Stephen Mcdaid on March 8, 2010
Interesting article, Jo. One of my worries is falling foul of 'accidental plagiarism' plot/premise-wise, because I read countless books as a youngster.
Jonathan Hopkins on March 8, 2010
I don't think you can plagiarise plot or Shakespeare's and Dickens' ancestors would all be multi-millionaires from the lawsuits - those two covered just about everything.
Story's a bit more difficult. If you put yourself into your characters, I don't see how you can avoid using past experience. And a lot of that will be dredged from the subconcious, where real and imaginary get mixed up. You just have to be convinced, in your own mind, that what you write is new and different. Quite frankly, readers won't want it if it isn't.
Of course there are some who think "Kookaburra sits in the old gum tree" is the same song as "I come from the land down under" - it takes all sorts. Especially if they're on a percentage. (Sorry - inner cynic got the better of me there).
I write about Peninsular War cavalry so made sure I avoided reading Allan Mallinson's cavalry novels, even though they cover a slightly later period. But I read plenty of other military stories, so did I plagiarise them? They cover the same years, will include the same battles, same regiments, same weapons etc etc. The answer is I know I have not.
But there was a Biggles book I read years ago with a sort-of similar story line....if I remember rightly(!)
Claire Fogg on March 9, 2010
uncleararthur - I know what you mean about Dickens and Shakespeare covering just about everything.
There have also been some truly bizarre approaches to the classics lately - have you seen the book Pride and Prejudice and Zombies? It was a viral hit and has sparked a new genre called mash-ups.
John Aston on March 9, 2010
And that is why this site is a part of my daily Rituale Romanum: email, BBC News, YouTube and then W & A. The truly modern liturgy, I think. :-) Thanks Jo, that’s very helpful, (*tipping imaginary hat to ya’.)
... Just out of curiosity, bearing in mind the pedants out there: I presume the ACTUAL word count (which you suggest is around a dozen words) or, I should say, the LIMIT allowed before it becomes prudent to seek permission, varies from publisher to publisher, agent to agent, and even: aggrieved author from aggrieved author, and is consequently subject to artistic sensibilities? ...
Keep up the great work, it's much appreciated.
J. J. Smith
Irene Cunningham on March 10, 2010
I was advised to eject the scattered lines of song lyrics from my MS because of copyright. How do we stand on this anyway? Would a publisher or agent think it all too difficult and rather they weren't there? Can we throw in the odd line? I don't think I went over ten words, and would definitely have made reference to the origins of the quotes.
Irene