By Shreyashi Sanyal and Ambar Warrick

British stocks ended lower on Monday after a series of strong sessions as heavyweight AstraZeneca slipped on a report that it had approached Gilead Sciences for a merger, while broader concerns over corporate debt also weighed.

The FTSE 100 slipped 0.2% and AstraZeneca shed 2.7%, serving as the biggest weight on the index. Bloomberg News reported on Sunday about the British firm's approach for a possible megamerger with its U.S. rival to form one of the world's largest drug companies.

The midcap FTSE 250 index fell 0.5%, with financials among the biggest drags. Healthcare stocks <.FTNMX4530> lagged the broader market, while oil and gas stocks <.FTNMX0530> jumped 1.7%, tracking recent gains in crude prices after the OPEC+ extended its production cuts. [O/R]

An industry body, TheCityUK, has said about a third of the debt being taken on by British firms under the government's emergency COVID-19 lending plans could be unsustainable, raising the need for fresh capital from new investors.

"The lobby group warned that up to 36 billion pounds ($45.67 billion) of recent government-backed loan initiatives risk turning toxic," said Chris Bailey, European strategist at Raymond James. "It's not an easy economic recovery to manage, even assuming no second wave."

British stocks have marked a meaty rise from lows hit in March, as investors looked to an economic recovery with the scaling back of novel coronavirus-driven curbs on social and business activity this month.

BP Plc rose 1% after Reuters reported that the oil and gas heavyweight will cut about 15% of its workforce in response to the COVID-19 crisis, which had decimated oil prices.

Plus500 Ltd slid 4.4% after the online trading platform said its quarterly revenue took a hit from client trading wins.

Oxford Biomedica climbed 4.8% after the gene and cell therapy firm said it signed a new manufacturing agreement to help it scale up production of AstraZeneca's potential COVID-19 vaccine to cater to demand in the United Kingdom and Europe.

(Reporting by Shreyashi Sanyal in Bengaluru; Editing by Arun Koyyur, Uttaresh.V and Ken Ferris)