Britain's FTSE slips, though posts fifth straight month of gains

* FTSE 100 down 0.6 pct

* Longest monthly run of advances since 2013

* WPP jumps after update

* Shire and Next fall ahead of earnings (ADVISORY- Follow European and UK stock markets in real time onthe Reuters Live Markets blog on Eikon, see cpurl://apps.cp./cms/?pageId=livemarkets)

By Kit Rees and Alistair Smout

LONDON, Oct 31 (Reuters) - Britain's top share indexretreated on Monday but posted a fifth straight month of gains,as advertising group WPP rose after reporting its third-quarterresults.

The blue chip FTSE 100 index was down 0.6 percent at6,954.22 points by the close, but up 0.8 percent for October.

Shares in WPP rose 4 percent and posted theirbiggest daily gain in four months. WPP, the world's largestadvertising group, reported results in line with expectations.

"We are encouraged by the positive nature of this morning'supdate and remain fundamentally positive on WPP's ability tocapitalise on a solid medium-term outlook for global advertisingspend," Roddy Davidson, an analyst at Shore Capital Markets,said in a note.

However, investors were less optimistic about someforthcoming earnings. The top fallers were Shire, whichreports results on Tuesday, and Next, whose results aredue on Wednesday, down 2.8 percent and 3.2 percent respectively.

The FTSE 100 index posted a fifth month of gains in a rowfor the first time since early 2013. The index has been buoyedby a rally in banks and mining companies.

Banks were boosted by last week's well-received earningsfrom Barclays and Lloyds, with the FTSE 350banking sector hitting its highest level for theyear.

October saw the FTSE 100 set a record high of 7,129.83points. A weaker pound has bolstered the index, which hasrallied about 10 percent since Britain voted in June to leavethe European Union. The cheaper pound helps the index'sinternational, dollar-earning firms.

Some analysts, however, were more cautious on the outlookfor British shares.

"(The FTSE) is still continuing the positive run of gainssince Brexit, so it's a Brexit bounce. But it's run into thatprevious record high ... and pulled back quite significantlyfrom there," said Jasper Lawler, market analyst at CMC Markets.

Investors, he added, were holding back before upcomingcentral bank interest rate decisions and the U.S. presidentialelection on Nov. 8.

"I think no one's really got the confidence to buy themarket up through into new record highs."

The more domestically exposed FTSE 250 index fell0.6 percent, and was down 1.8 percent for the month of October.It has been weighed down by a spate of profit warnings and bigdeclines from companies including Berendsen, Cobham, Senior and Keller. (Editing by Mark Heinrich)