Tens of thousands of Irish shareholders run the risk of being effectively locked into an investment in Verizon after ignoring an offer to sell their shares.

Almost 200,000 Irish investors hold shares in the US telecoms group. Almost all were originally investors in the flotation of Telecom Éireann in 1999.

The vast majority hold just a handful of shares in the group – about 145,000 hold 10 or fewer and many had complained about the cost involved in selling out of the company.

They received these shares when Vodafone – which bought Telecom's Eircell mobile business in 2001 – sold its US business to its joint venture partner Verizon in 2014.

Last October, the company wrote to shareholders offering a “low-cost” option to sell their shares. It warned that once the sale window closed, the remaining shares would be managed from the US where they will face US withholding tax and will be obliged to deal with the US office of share registrar Computershare in any further dealings.

They will also face foreign exchange charges as all dividend cheques and payment for shares traded will be made in dollars or be subject to a range of charges.

The changes come into force for any shares not tendered for sale by tomorrow.

More than 35,000 Irish shareholders hold just one share in the company – worth $50 yesterday. Selling it under the current offer will cost a shareholder €17. Once the window closes, the cost of trading through Computershare jumps to $25 minimum plus 12 US cents a share, as well as the cost of communication with the US to execute the deal.

So far, only a quarter of Irish investors holding a single share have contacted the firm to sell them. There are still 98,581 people holding 10 or fewer shares in the business. Of the roughly 100,000 investors who originally held three or fewer shares, less than 30 per cent have moved to liquidate their holding. After tomorrow, the current option will no longer be available.