The Government has promised action to deal with "too long" queues at Heathrow but blamed the rain for recent troubles and insisted claims of two-hour waits were a "wild" exaggeration.
Immigration Minister Damian Green launched a robust defence of performance at the UK's busiest airport as London Mayor Boris Johnson joined mounting criticism.
Mr Johnson said visitors were being given "a terrible impression of the UK" and called for action to halt the vital transport hub "gaining such a poor reputation".
Despite efforts by border chiefs to prevent passengers taking pictures of queues, social media sites were full over the weekend with complaints about excessive waits.
Labour, which forced Mr Green to the Commons to account for the problems, said maximum wait times had been breached 107 times in the first half of April.
Addressing MPs, the minister said the "vast majority" of passengers were processed quickly and that it was important not in any way to compromise border security.
Despite targets being missed on occasion, 99 per cent of UK and other EU nationals got through within the 25-minute maximum and 96 per cent of the rest inside a 45-minute limit.
And the longest recorded wait was 90 minutes, he added - hitting out at "wild" reports of much longer delays but admitting: "These times are too long.
"Over the weekend there were some breaches of acceptable waiting times.
"This was caused mainly by the severe weather leading to flight diversions and changing flight schedules and the bunching of arrivals.
"Our information shows that queueing times bore no resemblance to the more wild suggestions."
Mr Green said a number of measures were in hand to improve the service at Heathrow including a new central control room and mobile rapid-response teams to deal with unexpected rushes.
There would also soon be revised work rotas and shift patterns - as well as an influx of temporary staff to deal with extra arrivals expected for the London 2012 Olympics.
"Border Force will ensure that all immigration desks at Heathrow and other key ports and airports in the South East are fully staffed during peak periods over the summer," he said.
"While we maintain the right levels of security checks, we will always seek to improve performance."
But shadow immigration minister Chris Bryant accused the Government of failing to put proper resources into border security.
"The Government is running out of alibis. These figures show the Government hasn't given the Border Force enough resources to do the job properly and is displaying utter incompetence.
"They can't blame it on the weather when there were 107 breaches of their target in the first 15 days of April," he said.
And Labour MP Keith Vaz, the chair of the home affairs select committee, said there had been two-hour-plus queues which were "a serious embarrassment".
He called for a summit of all those involved which could be held at Heathrow itself.
Mr Johnson, one of the most senior Tory politicians, expressed his worries directly to Home Secretary Theresa May in a letter expressing "serious concern".
Earlier, it emerged that Heathrow had been ordered by the Border Force to stop handing out leaflets to passengers acknowledging the "very long delays" at immigration.
Amid increasing anger, airport operator BAA tried to defuse tensions with a leaflet apologising for the problems, accepting people arriving in the country "deserved a warmer welcome" and explaining how to complain to the Home Office.
But Marc Owen, director of UK Border Agency operations at Heathrow, has told BAA that the leaflets are "inappropriate" and that ministers would take "a very dim view".
In an email obtained by The Daily Telegraph, he said: "Please refrain from handing out (the leaflets) or I will escalate (the matter) with ministers who are likely to take a very dim view."
Mr Owen also told BAA to prevent passengers taking pictures in the arrivals hall, sparking Labour claims of a cover-up.