The Public and Commercial Services (PCS) union could take legal action against the government for drafting in the military to cover for striking workers at airports.
The Border Force staff walked out from December 23-26, with the second wave of strikes taking place on December 28-31 in a dispute over pay and conditions.
The action is taking place at Heathrow, Gatwick, Manchester, Birmingham, Cardiff and Glasgow airports, in addition to the port of Newhaven.
However, there have been few reports of any delays as hundreds of military personnel and civil servants were drafted in to cover striking workers.
Mark Serwotka, PCS general secretary, said the government was able to avoid delays only by using hundreds of military personnel who had no authority.
He told BBC Radio 4’s World at One programme that the “untrained personnel are not legally entitled to stop people who they suspect of criminal activity”.
“The price the government is paying to not have queues is that they are not discharging their legal responsibilities,” he told the programme.
“I suspect we may well see some post-strike action litigation around this question.
A Heathrow spokesman told the Telegraph: “Operations continue to run smoothly and the airport is operating as normal. The immigration halls are free flowing with Border Force and the military contingency providing a good level of service for arriving passengers.”
Gatwick is telling passengers to expect longer wait times at passport control but said most passengers will be unaffected by the strikes, with flights operating as usual during this time.
Manchester, Cardiff and Glasgow told the Telegraph that passport control was operating normally.