(Reuters) -London's FTSE 100 continued its slide for a third day on Thursday, as Britain began preparations for a general election on July 4 and data showed growth across businesses noticeably slowed in May.

The blue-chip FTSE 100 index slid 0.3% while the pound strengthened against the dollar at $1.2738. The mid-cap FTSE 250 was flat.

An upbeat quarterly earnings report by Nvidia was offset by the surprise call for a general election by Prime Minister Rishi Sunak that spooked investors, prompting a strategic shuffle as they weighed the ramifications of the potential scenarios.

This pushed the utilities shares to the bottom of the sectoral charts, while defensive stocks such as Unilever, BAE Systems and GSK gained.

"There's an element of caution and there tends to be a knee-jerk reaction and if you were concerned as an investor about the prospect of a Labour government, you may come out of some positions and start to de-risk," said Christopher Peters, trading floor manager at Accendo Markets.

Among stocks, National Grid was the top loser on the benchmark index with a 8.6% drop, after it said it would raise about 7 billion pounds ($8.90 billion) via a rights issue.

Wizz Air jumped 4.2% after the European low-cost airline forecast a higher annual profit.

Qinetiq soared 13.4% after the defence group said it will raise its forecast for FY25.

Hargreaves Lansdown surged 8.2% after the investment platform rejected a 4.67 billion pound ($5.94 billion) takeover proposal.

AJ Bell gained 10.5% after the investment platform reported its half-year results.

Gains of Hargreaves and AJ Bell raised the investment banking sector to a more than two-year high and a leader among FTSE 350 sectors.

Meanwhile, the Purchasing Managers' Index showed growth across British businesses has cooled and was softer than expected.

(Reporting by Pranav Kashyap in Bengaluru; Editing by Sohini Goswami and Krishna Chandra Eluri)

By Pranav Kashyap