The original founders of Skype initiated a legal action tonight, demanding billions of dollars in damages against the investment group that has just agreed to buy the internet telephony company from eBay for $1.9 billion (£1.2 billion).

In a move that casts serious doubt on the deal, lawyers for Janus Friis and Niklas Zennstrom issued a writ for copyright infringement against the new investors, including Silver Lake Partners, the private equity firm, and venture capital firms Andreessen Horowitz and Index Ventures.

Skype, which started in 2002, allows users to make calls from their computers to landlines, mobile phones and other computers.

At its heart is the peer-to-peer technology created by its founders, who also devised Kazaa, the file-sharing software.

EBay paid $2.6 billion for Skype in 2005 but did not buy the core technology, licensing it instead from Joltid, Mr Friis and Mr Zennstrom’s company.

After the pair left Skype, they allege, eBay executives began to alter the code used in the technology in breach of the licence.

The dispute is subject to a court case in London next year.

EBay has admitted that, without the technology, Skype could no longer function as a business.

Mr Friis and Mr Zennstrom are now seeking damages in California against eBay, Skype and the investment group for continuing to use to peer-to-peer technology without permission.

They are seeking an injunction to stop the alleged infringement.

Under the deal announced this month, eBay will retain 35 per cent of the company but has to pay 50 per cent of "any monetary judgment that is rendered following the closing of the transaction", according to its filing to the US Securities and Exchange Commission.

The filing states that completion of the sale is subject to, among other conditions, “the absence of any injunctions relating to certain specified litigation matters”.

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