A planned mass public sector strike on Nov. 30 is expected to cause long lines and delays for those traveling internationally into the UK.

Close to two million public sector employees, including UK Border Agency staff members, are expected to take part in the walk off. Workers hope to stop public sector pension reforms that would require employees to work longer and contribute more each month.  Ministers maintain the cuts are necessary as people are living longer and contributing less than needed to sustain the pension system.

Heathrow's chief operating officer Normand Boivin warned airlines of long lines and delays up to 12 hours in the immigration hall.

The delays at immigration are likely to be so long that passengers could not be safely accommodated within the terminals and would need to be held on arriving aircraft, Boivin said in letter. This in turn would quickly create gridlock at the airport with no available aircraft parking stands, mass cancellations or departing aircraft and diversions outside the UK for arriving aircraft.

To avoid this situation, Heathrow has asked airlines to halve the number of passengers they fly into the airport during the strike. By reducing the number of incoming international passengers, the airport is optimistic that it will fly on a normal operating schedule. Many of the other transportation unions are not involved in the strike.

Those flying out of London or traveling on domestic flights should remain unaffected by the UK Border Agency walkout. UK Border Agency expects to perform at less than 50% capacity during the strike.

Many airlines, including Emirates and Etihad have already cancelled several flights into the country Wednesday in preparation for the strike. Both British Airways and Virgin Atlantic are offering passengers the opportunity to change the date of their flight for free.  All travellers flying Wednesday should keep in contact with their airline for updated information.

Britain's second largest airport, Gatwick, has also encouraged airlines to rebook passengers coming into London. British Airways is allowing these passengers to rebook three days before and up to a week after Nov. 30.

UNISON general secretary Dave Prentis said Monday that the strike could be the largest since 1926, when an estimated 1.75 million walked off their job.

The action will go ahead. There is absolutely no chance whatsoever of any deals over the next couple of days, Prentis told Reuters.

UNISON represents approximately 1.4 million public service workers. The union encourages workers to take to the street outside of town halls and hospitals to speak out on pension fairness.