Ground handlers employed by firm Menzies, which operates at Heathrow, have begun a 72-hour strike.
The union expects the dispute to lead to disruption, delays and cancellation for flights leaving Heathrow Terminals 2, 3 and 4.
There have yet to be any advanced cancellations as a result of the strike, with a Heathrow spokesperson saying the airport was working as normal.
They said: “Affected airlines already have contingency matters in place and flight cancellations are not anticipated.”
The union has warned that the action could affect passengers travelling with airlines including Aer Lingus, American Airlines, Finnair, Qantas and Lufthansa.
Other airlines such as Air Canada, Swiss Air, Air Portugal, Austrian airlines and Egypt Air are also expected to be affected.
Unite general secretary Sharon Graham said: “Unite’s members at Menzies play a critical role in ensuring that planes operate safely at Heathrow.
"Menzies is a wealthy company and it can fully afford to pay its workers a decent pay increase. It is greed not need which is preventing a fair pay offer being made.
“Unite’s members at Menzies will receive the union’s comprehensive support.”
Unite says workers have been pushed to strike over "derisory pay offers to the ground handlers."
"All of the offers made to date are far below the current real inflation rate of 14.2 per cent (RPI) and amount to a substantial real terms pay cut, the union has said.
Menzies says it has come to a "positive position" with GMB Union members of the bag handling team who are now being balloted on an offer, but that Unite was being "obstructive".
Unite regional officer Kevin Hall said: “The strike action will inevitably cause serious delays for passengers at Heathrow but this dispute is entirely of Menzies own making.
"It has had every opportunity to make our members’ a fair pay offer, but it has stubbornly refused to do so.”
Some passengers flying into Heathrow early this morning said they had been warned not to bring hold luggage.
"Got an email from American Airlines saying they recommend taking only carry on because the baggage handlers are going on strike at Heathrow," one person tweeted. "This makes me sad, was planning to do all my Christmas shopping in London. I won’t have room for shopping with only my carryon."
Before travelling into or out of Heathrow today and over the weekend, it may be worth checking with your airline for their advice regarding luggage.
Strikes scheduled to take place involving workers at Dnata were called off earlier this month after members accepted an improved offer.
Strike action involving Menzies workers employed in its cargo and trucking sections has been suspended to allow the workers to be balloted on a significantly improved offer.
Due to the Dnata and Menzies Cargo strike not going ahead, passengers with Qatar Airways going to the World Cup will not be directly affected by the dispute.
Miguel Gomez Sjunnesson, EVP Europe for Menzies Aviation, said: “Following further discussions this week regarding pay increases for our Heathrow ground handling workforce, we are pleased to have reached a positive position with GMB who represent the majority of this workforce. As a result, GMB members are being balloted with the view that the best possible outcome has been reached.
"Unfortunately, the same cannot be said for Unite who continue to be obstructive and push for unnecessary strike action rather than provide any realistic counter offers for consideration, which will benefit no-one and only delay the process.
"We remain ready and willing to meet with Unite to hear their concerns and discuss the deal we have agreed with GMB in more detail, which we believe is in the best interest of our employees.
"I want to reassure our airline customers and their passengers that we have robust contingency plans in place should Unite elect to continue with unnecessary industrial action, and we are confident our operations will not be impacted.”
Heathrow has been contacted for comment.