CBI figures show a third of companies employing more than 5,000 staff expect a claim for bargaining rights in the coming year. This compares with 13 per cent overall and less than one in 10 firms with fewer than 200 staff.
Dominic Johnson, head of employee relations, said this is because the need to show majority support from staff means unions will target companies where they are already recognised in one part of the business or where they have a history.
According to the CBI's latest employment trends survey, the biggest companies are five times more likely to recognise a union than those employing fewer than 50 people.
Even where unions have been de-recognised many employees are still members giving the union a good starting point, Johnson said.
The TUC said it is too early to say which companies will be targeted, but large and small businesses are likely to be affected.
"We would assume that there will be recognition agreements in a variety of companies, both large and small, the only ones that won't be affected are those with less than 20 people, because the law does not cover them," said a spokesperson.
"All workers are entitled to a voice at work, regardless of the size of the company."
Johnson said, "Small companies are less likely to face recognition claims in the next 12 months, but no company should expect not to be approached."
• Staff at Virgin Atlantic are being asked to vote on whether they want to be represented by a union. A secret ballot of the 5,000 staff will be held next week, according to a report in The Guardian.