LONDON (Reuters) - GlaxoSmithKline has won another reprieve for its blockbuster Advair lung drug after U.S. regulators delayed approval of a generic copy from Novartis’s Sandoz division.

Signage for GlaxoSmithKline is seen on its offices in London, Britain, March 30, 2016. REUTERS/Toby Melville/File Photo

The Swiss drugmaker said in an emailed statement on Thursday it had received a so-called complete response letter from the U.S. Food and Drug Administration (FDA) and a generic Advair launch this year was now “highly unlikely”.

Such letters typically outline concerns and conditions that must be addressed to gain U.S. approval.

The setback follows similar delays last year for rival copies of GSK’s inhaled drug developed by Mylan and Hikma, which has been working with Vectura.

After Mylan and Hikma’s problems, most industry analysts expected the generic threat had been pushed back until mid-2018, providing Britain’s biggest drugmaker with a short-term profit boost. But the Sandoz news may cause a rethink.

“It now looks like the can has been kicked down the road,” Berenberg analysts said in a note.

Advair is particularly difficult for generic drugmakers to copy because they must develop an inhaler device that delivers the medicine correctly into patients’ lungs.

The fact that Sandoz, one of the world’s most experienced generics firms, has also stumbled highlights the challenges of making a generic that is acceptable to the FDA, although such Advair copies are already available in Europe.

The timing of the arrival of generic Advair in the United States is critical to GSK’s near-term earnings outlook. The drugmaker said on Wednesday that earnings, at constant exchange rates, would be flat to down 3 percent in 2018 if generic Advair launches in the U.S market by mid-year. Without generics, earnings would be up 4 to 7 percent.

A GSK spokesman had no immediate comment on the latest Sandoz news.

Dealing with the threat to Advair, which has generated more than $1 billion in annual sales since 2001, is a key challenge facing GSK’s core pharmaceuticals division.

Another is the competitive pressure in its big HIV medicines division, where Gilead Sciences on Wednesday won a FDA green light for a once-daily, triple-combination tablet containing a new integrase inhibitor bictegravir.

GSK’s majority-owned ViiV Healthcare unit hit back by filing a lawsuit alleging that Gilead was infringing patents on ViiV’s rival integrase inhibitor dolutegravir.

ViiV is not looking to have Gilead’s product removed from the market but it wants compensation, which analysts said could result in royalties being paid to ViiV by Gilead, if the legal action is successful.

GSK shares were 2.7 percent higher by 0910 GMT.