US authorities have intensified an investigation into Tesla's privatisation plan by issuing formal summons to the electric car manufacturer's chief executive Elon Musk.

The Securities and Exchange Commission (SEC) investigation was launched after Mr Musk tweeted on August 7 that he had “funding secured” to take the business private at a price of $420 per share, triggering a 13 per cent spike in the share price of Tesla.

The SEC investigation into the company stepped up a gear on Wednesday when the SEC issued a formal subpoena to Tesla officials and Mr Musk, the company's billionaire founder, according to Fox News.

The SEC has reportedly demanded more information regarding the statement that Tesla had "funding secured" and the company's privatisation plan, which were first published on Twitter.

Mr Musk subsequently clarified that the funding would come from the sovereign wealth fund of Saudi Arabia.

Tesla's share price dropped 3.9pc following the reports of the subpoenas, which are likely to complicate any discussions about taking the group private.

The news follows Tesla's decision to create a special committee to consider a proposal from Mr Musk to take the business private.