UPDATE 1-FDA delays GSK, Genmab leukaemia drug ruling 3 mths

* Arzerra ruling delayed to consider additional data

* Genmab shares fall 7 percent (Adds details)

LONDON, June 16 (Reuters) - Britain's GlaxoSmithKline (GSK.L) and Denmark's Genmab (GEN.CO) said on Tuesday that U.S. officials have delayed the deadline by which they expect to rule on their leukaemia treatment Arzerra by three months.

The U.S. Food and Drug Administration (FDA) has extended the deadline to give it time to review additional chemistry and manufacturing data.

The announcement sent Genmab shares down by 7 percent, but left Glaxo's shares largely unchanged.

Glaxo and Genmab are seeking accelerated approval of Arzerra, also known as ofatumumab, based on results from a small trial that did not include a comparison arm.

The companies were granted priority review by the FDA at the end of January, and the latest delay means that the FDA will decide on the application by October 31.

The drug targets chronic lymphocytic leukaemia, a common type of blood cancer in adults.

London-based Glaxo bought the global rights to Arzerra in December 2006 in a deal worth up to $2.1 billion, a record sum for a biotech product agreement at the time. (Reporting by Ben Deighton, editing by Hans Peters)