Many commentators are suggesting that a union cannot conduct a single strike ballot across different employers, known as an "aggregated" ballot.

As the law currently stands, a union can do so provided (a) there is a valid trade dispute affecting its members within each affected employer; (b) that dispute is common to all the affected employers; and (c) the complex rules on aggregated ballots are satisfied.

The consequences of aggregated ballots for the affected employers are drastic; they have no means of knowing how their own employees voted, and they are bound by the overall result. So an employer with good employee relations can get caught up in a strike over which it has little control. However, as always, the full picture is more complicated.

The industrial relations clout provided by an aggregated ballot comes at a price. The union must tread carefully to ensure that the same trade dispute applies to all affected employers, which will be difficult with a generalised complaint about pay, perhaps less so with a specific demand, particularly amongst employers within an industry or government sector.

Moreover, the rules on aggregation are complex and easily broken, in which case the whole ballot becomes unlawful. Our specialist industrial relations team frequently advises employers that aggregated ballots have broken the rules, with the result that many unions now prefer to conduct numerous separate workplace ballots rather than risk everything on a single aggregated ballot.

Of course, there is nothing to stop a union achieving a similar impact to an aggregated ballot by conducting separate but simultaneous ballots amongst its members in several employers, and to co-ordinate the timing of any industrial action.

The big question is whether the current febrile climate will encourage unions to risk aggregated ballots, potentially offering them enhanced clout, or whether they play safe and hold separate workplace ballots.