LONDON, Jan 25 (Reuters) - A shift by European Union markets to halving the time it takes to settle stock trades is a matter of when, rather than if, given that the benefits will outweigh the short-term challenges, the bloc's financial services chief said on Thursday.

Wall Street is shifting from settling stock trades over two business days to one business day in May, piling pressure on European markets to follow suit.

"The question is no longer if, but how and when it will happen in the EU," EU financial services commissioner Mairead McGuinness told a conference in Brussels.

(Reporting by Huw Jones; Editing by Alison Williams )