The leader of the Labour Party's biggest single union benefactor demanded an investigation yesterday by Tony Blair into alleged interference in his organisation's internal affairs by a minister.

Derek Simpson, joint general secretary of Amicus, wrote to the Prime Minister asking for an urgent meeting to discuss apparent "meddling" by John Spellar, minister of state in the Northern Ireland Office. Mr Spellar was a leading supporter of Sir Ken Jackson, the right-winger who failed to be re-elected as general secretary of the union's engineering section, a ballot right-wingers had tried to rig.

Mr Simpson's move follows the emergence of a controversial letter, dated 10 December and purporting to be from Mr Spellar to Pat McFadden, director of political operations in Downing Street.

Both men deny knowledge of the communication. It suggests the minister has continued his involvement in the right-wing faction at Amicus, despite an earlier assurance by the Prime Minister to Mr Simpson last summer such interference would not be tolerated.

Mr Simpson, a left-winger, is also expected to draw Mr Blair's attention to a letter dated 10 July urging union activists to attend a meeting of a new right-wing Amicus faction at Brandhall Labour Club in Oldbury, the address of Mr Spellar's Warley constituency offices. The name of the new right-wing grouping is Members First and the minister is heavily involved in a party faction called Labour First, which has a similar political outlook.

Mr Simpson, who is expected to raise the matter with Mr Blair next Tuesday at the annual TUC conference in Brighton, believes the Northern Ireland minister is trying to destabilise his leadership.

Mr Spellar's political office said the minister, now in France, did not "recognise" the letter allegedly sent to Mr McFadden. A spokeswoman said the signature did not look like Mr Spellar's.