Trafalgar Square's fountains have been shut down after soaring temperatures caused the growth of unsightly algae.
A Greater London Authority spokesman said the chlorinating unit in the fountains broke down in the heat and the capital's hot weather created the conditions for the algae to grow.
Workers drained the water from the fountains today and began clearing the algae before fixing the unit and opening the fountains again in time for the weekend, he said.
The slimy green growth was noticed by maintenance staff earlier this week, when temperatures reached 30C (86F).
The Grade II-listed fountains, which include bronze statues of mermaids, a merman, dolphins and Tritons, will be ready in time for the Pride Parade on Saturday, the spokesman said.