London 2012 is withdrawing half a million Olympic football tickets from sale but 200,000 for other sports are to be made available.
Organisers are to reduce capacity at Hampden Park in Glasgow and Cardiff's Millennium Stadium.
Cardiff's upper tier of seats will be closed for some games.
But 200,000 more contingency tickets will be made available for other sports, as well as the 50,000 non-football tickets yet to be sold.
The news comes as athletes and officials have been arriving at the Olympic Village, with Heathrow Airport experiencing its busiest day on record on Monday, and the first priority "Games Lane" now in operation.
BBC sports news correspondent James Pearce said it was a "fairly surprising revelation from London 2012" that so many tickets had yet to go on sale.
Organisers said there had been around one million football tickets left but these have been cut in half by reducing capacity at stadiums.
A spokesman for Locog said: "We are planning to reduce capacity across the venues by up to 500,000 tickets across the tournament.
"This will involve possibly not using a tier, or an area of a ground, in some of the venues.
"This can apply to men's and women's football, if necessary."
The Olympic Games begin with a round of football matches on Wednesday 25 July, including Team GB's women taking on New Zealand in Cardiff, two days before the opening ceremony in Olympic Park.
The capacity in the stadium will be scaled down from nearly 75,000 to 40,000.
London 2012 chairman Sebastian Coe told the BBC "just shy of 40,000" tickets had been issued for that match and suggested the shortfall in football ticket sales was to be expected.
"It's always a challenge; you've got lots of venues and lots of matches and you don't know until late on who is going to be playing," he said.
Lord Coe said some of the remaining tickets may be issued using London 2012's Ticket Share system, which provides free tickets to children, servicemen and women and their families, and sportspeople, funded through hospitality packages.
"We've got millions of tickets into the hands of British people. That's exactly what we said at the beginning of the process," Lord Coe told James Pearce.
"Nearly two million people applied for tickets. There was a massive demand but we were always clear that more tickets would come available late on."
Tickets are still available for sports including athletics, hockey and beach volleyball, as well as in the higher price bands for the opening and closing ceremonies.
London 2012 organising committee Locog has struggled to sell football tickets outside on London, amid disagreements with the national governing bodies in Wales and Scotland over selection for the Great Britain teams.
Officials fear their right to compete as individual home nations in other major tournaments could be compromised if they join Team GB.
Ticket sales opened in March 2011 and were beset by technical difficulties. During the second round of sales, thousands of people thought they had bought tickets only to be told the following day they had been unsuccessful.
At a media briefing on Tuesday Lord Coe also said that the wet weather was "a problem and is causing us extra challenges" in particular at the equestrian venue of Greenwich Park and the Eton Dorney rowing and canoeing venue.
He advised spectators to "bring wellies" to cope with the saturated ground.