• by Steve McClellan  @mp_mcclellan, February 11, 2013

With WPP’s stock on an upswing, investment firm Black Rock has increased its ownership stake in WPP to more than 5.1%, the UK agency holding company confirmed Monday.

The investment would appear to make Black Rock the largest single shareholder in the company. WPP confirmed Black Rock’s share purchase in a regulatory filing but declined to confirm whether the investment company is now the largest single shareholder.

However, in its 2012 annual report, WPP said it was aware of just one shareholding interest with a stake of 3% or more and that was investment firm Legal & General, with a 3.55% holding in ordinary shares. “The disclosed interest refers to the respective combined holdings of this entity and to interests associated with it. The Company has not been notified of any other holdings of ordinary share capital of 3% or more,” WPP stated in the report.

The Black Rock holding is substantially higher. According to Monday's regulatory filing, Black Rock now holds voting rights in more than 64.7 million shares, or  5.12% of the company.

The Black Rock investment comes as WPP stock is surging -- it reached $15.70 on the London Stock Exchange Monday, above the 10-year high-water mark for the stock, the company confirmed.

The share resurgence comes amid reports that the market research business -- which WPP has a sizeable stake in via its Kantar arm -- is showing signs of improvement, particularly in Western Europe where the industry has been struggling.

Also, about two weeks ago, investment house Jefferies upgraded WPP to a “buy,” citing a positive outlook for 2013 budgets for some of the holding company’s key clients, such as Unilever. With client spending that “looks encouraging,” the firm stated in a research note, “with expectations low, we see upside risk.”

That said, WPP executives have downplayed expectations for 2013 in recent presentations.

“The watchword is caution,” Company CEO Sir Martin Sorrell told attendees at the UBS media conference in December. He added that WPP could probably achieve 2013 organic revenue growth of between 2% and 3%.   WPP is set to report full 2012 results on March 1.