LONDON, Aug 2 (Reuters) - British and Dutch wholesale gas prices were mixed on Tuesday due to ongoing uncertainty about Russian gas supplies but strong liquefied natural gas (LNG) send-out.

The Dutch gas contract for next-day delivery was up 4.80 euros at 203.60 euros per megawatt hour (MWh at 0913 GMT, while the contract September delivery rose by 6.50 euros to 206.00 euros/MWh.

Physical flows via the Nord Stream rose slightly on Tuesday but uncertainty over flows - now running at only 20% of total pipeline capacity - keeps market players worried over Europe's ability to build storage ahead of winter.

"Reshuffled LNG flows, rising UK and Norway supplies, and softer demand have put north-west Europe storage balances in a good position seasonally," said analysts in a BofA global research report.

"Yet NS1 will need to operate back at 40%, to push NWE storage levels near the 5-yr average to start winter. If Russia cuts NS1 to zero again, gas inventories would likely struggle to reach the 2021 highs, and storage at season end could fall to perilously low levels," they added.

In the UK market, the day-ahead contract was down 5.00 pence at 260.00 pence per therm, while the contract for weekend delivery was 13.00 pence lower at 250.00 p/therm.

The UK system was more than 11 million cubic meters (mcm) over-supplied on Tuesday, according to the National Grid website, despite an expected drop in natural gas production on the UK Continental Shelf (UKCS) by 23 million cubic metres (mcm) per day to 79 mcm due to multiple planned outages.

Stronger wind power also helped soften gas-for-power demand. UK peak wind power generation is forecast at around 14.7 gigawatts (GW) on Tuesday, falling to 12.9 GW on Wednesday, out of a total metered capacity of 19.9 GW, Elexon data showed.

In the European carbon market, the benchmark contract rose by 0.226 euros to 80.78 euros a tonne.

(Reporting by Marwa Rashad, Editing by Nina Chestney)