Chief executive Kevin Lagan was sent the crude warning in a package to the firm’s Belfast office with a note stating: “Quinn … is this what you want”.

Lagan told the BBC that the bullet was an attempt to intimidate the firm over its recent discussions about a possible joint venture with Quinn Building Products.

Lagan Cement Group is in talks with Quinn Building Products about combining their cement and building products businesses based in Ballyconnell, Derrylin, Kinnegad, Belfast, Cork and Benelux.

The Quinn Group, which was previously owned by Fermanagh businessman Sean Quinn now decalred bankrupt with debts of 2bn euros, was taken over by a consortium of banks in April 2011.

Since then there have been several attacks on Quinn Group property.

Lagan said: “This is clearly an attempt to intimidate myself and the Lagan Group at a time when we are engaged in discussions with Quinn on combining our cement and building products businesses.

“The proposed joint venture is a positive move for everyone and will create a sustainable, independent cement manufacturer that can continue to support its customers and employees on a competitive basis.

“The people behind this crude, intimidatory tactic clearly aren’t interested in protecting jobs, but we will not be swayed from our determination to complete our discussions successfully.”

If the plan goes ahead it will be the second largest cement supplier in Ireland behind Irish Cement, which is owned by the multinational CRH.