UK stocks opened slightly lower on Friday morning head of a crucial weekend for Greece, as the government holds a referendum on European bailout proposals.With Friday's economic data calendar looking sparse, investor sentiment was likely to be driven by developments in Athens, as the latest polls suggest that the result of Sunday's vote is too close to call.The FTSE 100 was trading 0.22% down at 6,615.71 early on, with RBS and a host of mining stocks providing a drag.The Greek government has confirmed that a referendum will go ahead as planned on Sunday, while creditors have refused to discuss a bailout deal until after the public vote."Yet there is still a chance that the referendum might not go ahead, with the Greek Council of State to vote whether it breaches the country's constitution. Whilst the body is likely to give it the all clear, it adds another shade of uncertainty to a situation defined by its instability," said Spreadex analyst Connor Campbell.According to the head of the Hellenic Chambers of Commerce, Constantine Michalos, cash reserves at Greek banks were down to just €500m. Although banks were expected to reopen next week, Michalos said: "Anybody who thinks they are going to open again on Tuesday is day-dreaming. The cash would not last an hour."The Markit/CIPS UK services purchasing managers' index is likely to be the main event of the day for the City and comes on the back of mixed PMIs on the construction and manufacturing sectors earlier in the week. Analysts expect the PMI to have risen to 57.5 after falling to a five-month low of 56.5 in May.Meanwhile, equity markets in the States will be closed for a long weekend as the country holds its Fourth of July celebrations.RBS and mining stocks lead the fallRoyal Bank of Scotland was in the red on reports that it faces a $13bn claim over allegations regarding its actions before the 2008 crisis. The bank was involved in a long running case in a US court over how it packaged mortgage bonds and sold them to government lenders overseen by the Federal Housing Finance Agency.Mining stocks were also out of favour, with BHP Billiton, Anglo American, Fresnillo, Rio Tinto and Glencore all trading lower.Defence group BAE Systems was making gains after UBS upgraded the stock to 'buy' from 'neutral' but cut the price target to 510p from 540p.BT edged lower after Ofcom said that it has fined the telecom group's takeover target EE £1m for failing to comply with its rules on handling customer complaints.Market MoverstechMARK 3,131.21 -0.10%FTSE 100 6,615.71 -0.22%FTSE 250 17,649.17 -0.21%FTSE 100 - RisersTUI AG Reg Shs (DI) (TUI) 1,058.00p +1.44%Weir Group (WEIR) 1,749.00p +1.33%BAE Systems (BA.) 458.70p +1.19%RSA Insurance Group (RSA) 415.30p +1.05%Ashtead Group (AHT) 1,069.00p +0.94%Pearson (PSON) 1,226.00p +0.91%BP (BP.) 441.25p +0.88%International Consolidated Airlines Group SA (CDI) (IAG) 492.90p +0.76%Sage Group (SGE) 514.00p +0.69%Randgold Resources Ltd. (RRS) 4,236.00p +0.67%FTSE 100 - FallersVodafone Group (VOD) 231.90p -1.70%BHP Billiton (BLT) 1,252.00p -1.61%Standard Chartered (STAN) 1,037.50p -1.52%Royal Bank of Scotland Group (RBS) 361.00p -1.39%Anglo American (AAL) 907.50p -1.17%Fresnillo (FRES) 680.00p -1.16%Rio Tinto (RIO) 2,603.00p -1.14%Rolls-Royce Holdings (RR.) 866.50p -1.14%Barclays (BARC) 263.55p -1.07%Glencore (GLEN) 253.05p -1.06%FTSE 250 - RisersAuto Trader Group (AUTO) 313.60p +2.38%IMI (IMI) 1,177.00p +2.35%Renishaw (RSW) 2,285.44p +1.35%Cobham (COB) 265.70p +1.33%Vesuvius (VSVS) 432.10p +1.17%Ocado Group (OCDO) 464.00p +1.11%Rightmove (RMV) 3,287.00p +0.89%AO World (AO.) 139.40p +0.72%Telecom Plus (TEP) 1,009.00p +0.70%Indivior (INDV) 228.90p +0.66%FTSE 250 - FallersZoopla Property Group (WI) (ZPLA) 254.30p -3.71%FirstGroup (FGP) 116.40p -2.84%Tullow Oil (TLW) 341.30p -2.49%Serco Group (SRP) 127.80p -2.29%Just Retirement Group (JRG) 174.00p -1.97%Jimmy Choo (CHOO) 167.80p -1.87%Fidelity China Special Situations (FCSS) 149.30p -1.78%Premier Oil (PMO) 147.10p -1.74%Computacenter (CCC) 754.00p -1.57%Nostrum Oil & Gas (NOG) 590.50p -1.34%