Amicus, a major union in the UK, is to discuss a possible merger with another union based in the US and Canada.
If agreed, it would bring another 1.2m members to Amicus, creating one of the biggest unions in the world with more than 3m members.
Amicus General Secretary Derek Simpson is due to meet officials from United Steel Workers in about a fortnight.
The BBC's Stephen Cape said it was the first time such an international merger has been mooted.
Pay strikes
He said it was a reflection of the changing world, where union influence was in decline yet there was a growing need to fight global companies.
Although there are different labour laws in other countries, a super union could put pressure on companies and their managers, he added.
On some issues, for example pay, it would be possible to co-ordinate strikes at a company with plants in Britain and America.
Amicus is already set to expand next month when it merges with the Transport and General Workers Union (T&G), making it the country's biggest union.
Previously, Mr Simpson has spoken of creating "a single global trade union movement capable of challenging the might of multinationals".
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