By Shreyashi Sanyal and Shashank Nayar

UK stocks slipped for the fourth straight day on Thursday, with the FTSE 100 index marking its longest losing streak in three months, hurt by fears of a new wave of COVID-19 infections in the United States and a bleak economic outlook by the Federal Reserve.

Total cases of the novel coronavirus in the United States surpassed 2 million on Wednesday, while an expert said the country may see 200,000 deaths at some point in September.

Furthering worries was the Fed's projection of a 6.5% contraction in the U.S. economy in 2020, while signalling plans of years of extraordinary support for an economy facing a long road back from the COVID-19 pandemic.

"The pessimistic outlook from the Fed was not unexpected, but right now investors are looking at potential declines further because markets are spooked by the possibility of a second wave of coronavirus infections," said Joshua Mahoney, markets analyst at IG Group.

The blue-chip FTSE 100 index closed 4% lower. The British mid-cap index fell 3.6%, logging its worst day in nearly two months.

British stocks are now set for their first weekly fall in four, with hopes of a quicker economic revival fizzling out as preexisting worries of the COVID-19 pandemic hammer risk appetite.

"In the UK, we have gone from a position from market optimism to a position where clearly, we are not going to see that kind of drive anymore," Mahoney said.

Travel and leisure stocks took another beating on Thursday, with Carnival's 12% slide leading declines on the FTSE 100 index. EasyJet and Cineworld also ended lower.

Unilever proposed to ditch its dual Anglo-Dutch legal structure and create a single company in Britain to give it more flexibility for mergers and acquisitions. UK-listed shares of the company fell 1%.

(Reporting by Shreyashi Sanyal and Shashank Nayar in Bengaluru; editing by Uttaresh.V and Angus MacSwan)