Shares of electric carmaker Tesla were down ahead of their debut on the S&P 500 today, as worries over a fast-spreading new coronavirus strain in Britain weighed on markets. 

Tesla's shares were recently down around 4% in pre-market trading after closing at a record high of $695 on Friday. 

The company, headed by billionaire Elon Musk, will become the most valuable ever admitted to Wall Street's main benchmark and will account for 1.69% of the index, according to S&P Dow Jones Indices' analyst Howard Silverblatt. 

The shares have surged some 70% since the middle of November, when Tesla's debut in the S&P 500 was announced, and have soared 700% so far in 2020. 

Tesla's addition to the S&P 500 led index-tracking funds to buy $90.3 billion of shares by the end of Friday's session so that their portfolios reflected the index, according to Silverblatt.

The change is effective prior to the open of trading today, S&P said earlier in December, and Tesla is replacing Apartment Investment and Management. 

Silverblatt said that for every $11.11 Tesla moves, the S&P 500 changes 1 point, while the S&P's 2021 price/earnings ratio will rise to 22.6 from 22.3. 

California-based Tesla's stock surge has put its market value at about $660 billion, making it the sixth most valuable publicly listed US company with many investors viewing it as wildly overvalued. 

Tesla is by far the most traded stock by value on Wall Street, with $18 billion worth of its shares exchanged on average in each session over the past 12 months.

This easily beat Apple into second place with average daily trades of $14 billion, according to Refinitiv. 

About a fifth of Tesla's shares are closely held by Musk, the chief executive, and other insiders.