Former cellphone market leader Nokia has had its credit rating downgraded from BBB+ to BBB by rating agency Standard & Poor’s.

S & P says the action reflects the continued erosion of Nokia’s smartphone market share, which has been falling since the announcement in February that Nokia was dropping its Symbian OS in favour of Microsoft’s Windows Phone platform.

With customers are becoming increasingly likely to choose a smartphone by OS than by manufacturer, Nokia’s move to a branded operating system made sense; however, it has struggled to bring devices to the market in a timely fashion.

As recently as March, S & P rated Nokia’s credit an A.

The agency does expect a resurgence when the Windows-enabled phones start to hit the market, saying revenues are expected to return to 2010 levels by 2013.

Read our story on Nokia's latest financial results here.