After the market's close, Comcast class K shares will be replaced on the S&P 500 index by its class A shares. The difference between the two classes is voting rights - only class A shareholders can vote.

The S&P 500 index allows for only one class of shares from each company. Comcast had listed its class K shares as it had been the bigger class by market capitalisation with 914m shares compared to 22m class A shares. However, following the merger, AT&T shareholders received class A shares, which increased the number of such shares to 1.24bn.

Leo Guzman, president of brokerage company Guzman & Company, tracks index funds. He said the removal of class K shares in the S&P 500 index for class A shares would sharply widen the spread between the two class of shares.

Under the terms of the complex transaction, AT&T is exchanging its broadband cable operations in return for Comcast stock with a value of about $30bn.

On completion of the deal, AT&T will distribute those new Comcast shares to its shareholders in a ratio of approximately 0.32 new Comcast shares for each existing AT&T share. Investors in AT&T will hold two pieces of paper - their existing AT&T shares and shares in the enlarged Comcast.

The combined value of those two shareholdings will be the same as the value of AT&T shares today. In nominal terms, however, AT&T stock will trade at a much lower price as it represents a share of the company's telecommunications business and no longer includes the cable operations.

To counter this, AT&T is planning a one-for-five stock consolidation after the deal closes, which will ensure that, based on current share prices, its stock trades at more than $25 rather than between $5 and $6.

Index fund managers, who own about 10 per cent of the class K shares, will have to sell their shares on Monday and buy class A shares, which could dramatically increase the premium. The change is also expected to spark selling from institutional and retail investors.