Mrs Foster said she believed the move by the "UCUNF will split the unionist vote at a General Election by standing in every constituency".

In an interview with the News Letter, Sir Reg said: "I'm not in any way moving away from the position that has been stated (by David Cameron]."

Sir Reg said that from his party's tallying of last week's poll, the UUP – which has only one MP to the DUP's nine – was ahead or level with the DUP in "nine or 10" Westminster constituencies.

Mrs Foster said this was a "shameful decision for Reg to preside over and shows that the Tories are leading Reg by the nose".

Mrs Foster also branded as 'nonsense' the tally figures being circulated by the UUP suggesting that they had a massive bounce in the European election in several constituencies.

Mr Foster said: "It is deeply regrettable that Reg Empey would deliberately choose to divide unionism on the basis of flawed UUP election tallies and a dislike of the DUP.

"Whilst David Cameron has made clear his intentions to run candidates in all 18 Northern Ireland constituencies at a General Election, I would have hoped that Reg Empey could have expressed his desire for some degree of unionist cooperation in particular areas."

She added: "The Ulster Unionist Party is clearly placing its electoral aspirations on the party's flawed European Election tallies. Many of these tallies have been viewed by DUP members and it is clear that they neither represent an accurate reflection of unionism nor the unionist/nationalist balance in Northern Ireland."

Mrs Foster said: "I would appeal directly to the many Ulster Unionist Party members and supporters across Northern Ireland who do believe in the benefits of unionism standing together to ensure that their views are heard. A divided unionism is a weakened unionism and whilst we may have party political differences, there are necessary occasions when those must be set aside in the common goal of ensuring that unionism is advanced and strengthened."