Heathrow has warned it could see even fewer passengers in 2021 than last year as expensive testing requirements and restrictions hold travel back.
The airport said recent changes to the government's traffic light system were "encouraging".
But its losses continue to mount after fewer than four million people used the airport in the first half of 2021.
It came as Ryanair said "haphazard" UK rules continued to create uncertainty despite a recent surge in bookings.
The airline said most people were booking at the last minute amid concern that the traffic light system - which ranks destinations in terms of Covid safety - could be changed during their trips.
In results for the six months ended 30 June, Heathrow said its cumulative losses from Covid-19 had grown to some £2.9bn.
The airport, which was Europe's busiest before the pandemic, said its total passenger numbers in the period would have taken just 18 days to reach in 2019.
It now believes 21.5 million passengers will travel through Heathrow in 2021, driven by what it hopes will be pent-up demand for holidays.
But that is down from 22.1 million in 2020, at the height of the pandemic, and almost 90 million in 2019.
"The UK is emerging from the worst effects of the health pandemic, but is falling behind its EU rivals in international trade by being slow to remove restrictions," said Heathrow boss John Holland-Kaye.
"Replacing PCR tests with lateral flow tests and opening up to EU and US vaccinated travellers at the end of July will start to get Britain's economic recovery off the ground."
Currently a UK citizen who visits an amber list destination such as Spain can avoid quarantine on their return if they are fully vaccinated, but they still need to pay for two private Covid tests.
Anyone not fully vaccinated - or who was fully vaccinated outside the UK - must quarantine on arrival and pay for three tests, conditions industry bosses say are prohibitive.
'More steady green lists'
Ryanair's chief financial officer, Neil Sorahan, told the BBC's Today programme that the EU had avoided the problem with its digital Covid certificate, which allows a person's vaccination status to be recognised across the whole bloc.
"I think the European policy has worked an awful lot better than in the UK. It's given more certainty, it's led to more steady green lists all across Europe. The UK has been a little bit more haphazard with what's on the green list, what's on the amber list."
The carrier said the EU certificate and scrapping of quarantine for vaccinated arrivals to the UK from mid-July had resulted in a surge in bookings over recent weeks.
It now expects to fly 90-100 million passengers this financial year - up from the lower end of a previously forecast range of 80-100 million.
The airline flew 27.5 million people in the year to March 2021, down from 148.6 million in the previous year.
Boss Michael O'Leary said: "Pricing remains below pre Covid-19 levels and there will continue to be great value for Ryanair guests travelling this summer as we focus on recovering traffic, jobs and tourism across our European network.
"Based on current (close-in) bookings, we expect traffic to rise from over 5m in June to almost 9m in July, and over 10m in August, as long as there are no further Covid setbacks in Europe."