Confidential talks aimed at creating a "super-union" covering huge swathes of the private sector are expected to begin this week.
A special meeting of the Transport & General Workers' Union (T&G) on Wednesday will approve a closer working relationship with the giant Amicus union, leading to a merger, according to the Independent.
That move, backed by the general secretary Tony Woodley, would create a union with more than two million members, ranging from unskilled workers to financial managers.
Amicus general secretary Derek Simpson has consistently argued in favour of one union for the private sector.
Although the executive of the GMB, the country's fourth biggest union, has so far rebuffed advances from the nascent organisation, it is thought it might be drawn into a merger. The construction union Ucatt will also be a candidate.
That would give the new union a membership of more than 2.7 million and call into question the continued existence of the Trades Union Congress (TUC) which acts as an umbrella organisation for the movement.
A spokesman for the T&G confirmed that there was a special meeting of the union's executive on Wednesday, but said the agenda was being kept under wraps.
The executive of the GMB has postponed a final decision on its future until after the union's national congress in June where delegates are likely to discuss amalgamation.
The ten biggest UK unions are:
- Unison - 1,301,000 members
- Amicus - 1,179,850
- Transport & General - 820,118
- GMB - 600,106
- Usdaw - 331,703
- PCS - 295,063
- Communication Workers' Union - 258,696
- National Union of Teachers - 239,796
- NASUWT - 223,486
- Ucatt - 110,886
The total membership of TUC-affiliated unions is 6.5 million.