Up to 1,000 jobs are at risk following the $8.6bn (£6.2bn) takeover of FTSE 100 cyber security company Avast by US rival NortonLifeLock.

The two cybersecurity companies confirmed plans to merge to markets on Tuesday night in a cash-and-shares deal that will see Avast delisted from the London Stock Exchange.

The chief executives of Avast and Norton, Ondrej Vlcek and Vincent Pilette, said the deal would allow them to combine their cyber products and better take on the challenge posed by Big Tech rivals and increasingly sophisticated hackers.

Founded in the 1980s, Prague-headquartered Avast develops cyber security products on a “fremium” model, giving away its technology for free and charging a premium for upgrades. It has more than 435m users and listed in London in 2018.

Arizona-based Norton is a cybersecurity pioneer best known for its antivirus products. Previously called Symantec, the consumer-focused business was formed after off its enterprise division to Broadcom for $10bn two years ago.

The companies said the merger would generate savings of about $280m. The combined companies will have revenues of $3.5bn and about 5,000 staff. Mr Pilette said he expected this number to fall to about 4,000 as duplication is eliminated.

Most of the jobs at risk in the US and in Europe. The deal is expected to take between 9 and 12 months to complete.

While the antivirus sector has come under scrutiny from regulators including the Competition and Markets Authority for its consumer practices, Mr Vlcek said he did not expect the deal to face too many hurdles.

“We have done our homework, we believe that the market is much broader today. It is clearly not just the traditional antivirus market with incumbents like Avast and Norton, but with Big Tech very active in the security area. We feel confident we will be able to explain that to regulators and get this through all the approvals that are needed,” he said.

Avast investors will have the option to take 90pc cash and 10pc in Norton shares, valuing their shares at 608p, or an offer of 69pc Norton stock and 31pc cash worth about 551p. The cash offer delivers a 20pc premium, analysts said.

Analysts at Jefferies said the deal appeared “strategically sensible” but investors may not feel the cash offer was particularly generous.

The founders and directors of Avast, who control 36pc of its stock, will back the deal. Mr Pilette will remain as chief executive of the combined company.

In results reported on Wednesday, Avast said revenues rose more than a tenth to $470m.

Avast shares rose 2.2pc to 581p, valuing the company at £6bn.