UK shares fall to 5-1/2 month low, supermarkets sink

* Weak exports a drag on UK growth, data shows

* Miners among worst hit

* Tesco (Xetra: 852647 - news) down after data shows fall in sales

* Standard Life (LSE: SL.L - news) slips after downgrade from RBC (Updates to add closing prices, comment)

By Sudip Kar-Gupta and Lionel Laurent

LONDON, June 30 (Reuters) - Britain's top equity index hit its lowest level since mid-January on Tuesday, pulled down by miners and supermarkets and underperforming even euro zone shares despite Greece being hours away from a repayment default.

While new data showed Britain's economy enjoyed a stronger start to the year than previously thought, buoyed by big-spending consumers and an upturn in business investment, weak exports continued to drag on growth.

The FTSE 100 index closed down 1.5 percent, with mining stocks BHP Billiton (NYSE: BBL - news) and Anglo American (LSE: AAL.L - news) down 3 to 4 percent after Australia cut its iron-ore price forecast for 2015 by 10 percent and metal prices slid on excess supply fears.

Retailers also fell after data from Kantar showed the overall UK grocery market fell 0.1 percent in the 12 weeks to June 21. Tesco, whose sales were estimated to have fallen 1.3 percent, saw its shares tumble 2.8 percent.

"Sentiment has been severely dented," said CMC Markets analyst Jasper Lawler.

Standard Life was among the worst performers, falling 3.1 percent after analysts at RBC (Other OTC: RBCI - news) downgraded the stock to "underperform" from "sector perform".

As Greece came closer to defaulting on a 1.6 billion euro ($1.8 billion) loan and at risk of sliding out of the euro zone , the FTSE 100 underperformed a 1.3 percent drop for top euro zone stocks as volatility surged.

"(Greece is) one of the biggest hurdles that the euro zone has faced. We could see the FTSE fall down to 6,400 points this week," said Harry Shann of Logic Investments. ($1 = 0.8953 euros) (Reporting by Sudip Kar-Gupta; Editing by Mark Heinrich)