​The battle for a better bra is heading to court.

Lululemon Athletica is suing Under Armour for allegedly copying a sports bra design, showing just how critical it has become for retailers to reinvent the ubiquitous undergarment.

According to Lululemon, its $US52 ($66.50) Energy Bra - which has four straps that criss-cross in the back - "does it all".

Which is why, the Vancouver-based company says, it's suing rival Under Armour for patent and trademark infringements. In its lawsuit, filed this month, Lululemon says it takes issue with four of Under Armour's sports bras, which range from the $US29.99 Armour Strappy to the $US39.99 Armour Eclipse Low Impact.

In the filing, Lululemon says "Under Armour's unauthorised acts have caused and will continue to cause irreparable damage to Lululemon and its business".

A spokeswoman for Lululemon declined to comment. A representative for Under Armour said the company "takes the intellectual property rights of others very seriously".

The two companies are fighting for a piece of the fast-growing sports bra market, which analysts say accounts for more than $US1 billion in US sales a year. Last year, Lululemon executives said third-quarter bra sales grew more than 20 per cent.

Patent lawyers say lawsuits of this type are rare in the fashion industry, mostly because few retailers are willing to go through the trouble - or expense - of securing design patents for their products.

"This is a long, expensive process - we're talking at least a year and half, and several thousand dollars - just to get the patent," said Laura Ganoza, an intellectual property lawyer in Miami. "That's a lifetime in the fashion industry, where the lifespan of an article of clothing is a season, if you're lucky."

Lululemon, which has more than three dozen design patents, "clearly sees this bra as a product that's going to have a long shelf life," she added. In order to secure the patent, the company had to prove that the sports bra's straps were an original, "non-obvious" design that has ornamental value (as opposed to being strictly functional).

"The bar for obtaining a design patent is high, which is one reason you don't see many cases like this," said Christopher Larus, an intellectual property lawyer in Minneapolis.

"If this case moves forward, I would expect there will be a fair amount of focus on whether Lululemon's designs are truly novel, or whether this is something that's been done before."

The bar for obtaining a design patent is high, which is one reason you don't see many cases like this.

Sports bras and bralettes have been driving much of the recent growth in the bra industry, which for years has been dominated by Victoria's Secret. But recently, the company's sales have been plunging, creating an opening for online start-ups, sportswear companies and others to step in.

"There's a realisation that consumers are looking for a new kind of bra - whether new fabrics or approaches to sizing - and companies are rushing to tackle that problem in one way or another," retail analyst Sucharita Mulpuru said.

"And it seems to me that Lululemon is trying to protect its designs and potentially scare off competitors."

This isn't the first time the company, which last year had $US2.34 billion in revenue, has taken a competitor to court. In 2012, Lululemon sued Calvin Klein for allegedly copying the waistband design on its $US98 Astro Pant. The companies later settled out of court for an undisclosed sum.

"What Lululemon is doing here is staking its turf," Jeremy de Beer, a law professor at the University of Ottawa, told Reuters at the time.

"The business strategy is to deter other people from even trying to copy designs, because it's going to cause them legal problems."

The Washington Post