24 is one of the shows planned for Sky's HD offering

The satellite operator's HD service is launching with channels showing films, sport and entertainment.

But many subscribers have been told they must wait several weeks for installation.

Sky said "more than enough" boxes were ordered but not all had arrived, and it was "very sorry" to let people down.

Anyone requesting installation on Monday, launch day, will be told that they must wait until early August, meaning they will miss the World Cup - which begins on 9 June - in HD.

The issue has surfaced on internet discussion forums with users saying their installation date has been moved from this week to June or July.

'Supplies to double'

But Sky said all customers whose installation date was changing were being offered a new date within three weeks of the original date.

"We're working hard to resolve the situation," said Robert Fraser, Sky's head of corporate press.

"Thousands of customers will have their HD box this week and many more will be installed in time for the start of the World Cup."

Those who had ordered first would receive their equipment first, he added, saying the quantity of set-top boxes delivered to the company was expected to double in the next three weeks.

By 1730 BST on Monday about 2,500 HD installations had been carried out.

Sky plans to show Star Wars: Revenge of the Sith in HD

Sky had previously said 40,000 advance orders were received in the first three weeks of bookings, which had opened on 13 April.

It had a total of 8.1 million subscribers in the last quarter of 2005.

HDTV promises higher quality pictures and audio through a compatible television set.

Sky is showing HD versions of its film and sports channels - Sky Movies and Sky Sports - plus the selected output of broadcasters such as the BBC.

The Star Wars movie Revenge of the Sith, US real-time drama 24 and live coverage of England's second cricket Test with Sri Lanka are planned for the first week of transmissions.

HDTV is also available on cable through the Telewest platform, and there is a limited trial through normal aerials for 500 selected Freeview users in the London area.

Wimbledon coverage

It may be several years before a full service can be offered via digital terrestrial because there is not enough space for the additional data contained in HD transmissions.

The BBC and ITV are promising to offer their coverage of the World Cup in the higher-quality format.

Other BBC shows to be transmitted in HD include re-runs of the Charles Dickens drama Bleak House and nature series Planet Earth, plus the Wimbledon tennis championships.

Several ITV dramas, including Agatha Christie's Poirot, will be available in HD, plus Jean-Michel Cousteau's wildlife documentary series Ocean Adventures.

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