LONDON, May 18 (Reuters) - European shares were set to open sharply lower on Friday, heading for their worst weekly loss since late November as ratings downgrades on Spanish banks and Greece's sovereign debt further heightened concerns about the euro crisis. At 0622 GMT, futures for the Euro STOXX 50 were down 1.3 percent while Germany's DAX futures were down 1.2 percent and futures on France's CAC were 0.9 percent lower. Financial spreadbetters expected Britain's FTSE 100 to open 60 to 68 points lower, or as much as 1.3 percent. Moody's Investors Service cut the long-term debt and deposit ratings of 16 Spanish banks, including the euro zone's largest Banco Santander, saying the government's ability to support some lenders had weakened.