The host nation are the largest team at the Games with 542 athletes and 812 support staff but the USA, the second largest, is expected to have more numbers at Danny Boyle's £30 million showpiece.
All 70 Team GB track and field athletes will be at their training camp in the Algarve while any athlete in action in the first three days of competition, such as three-time Olympic champion sailor Ben Ainslie and double gold medallist Rebecca Adlington, are expected to sit out the event.
Team officials will announce their flag bearer on Monday after a vote being held across all athletes, with four-time Olympic gold medallist Sir Chris Hoy the favourite for the honour, with his events not starting until the second week of the Games.
"We set down a clear set of guidelines regarding the opening ceremony some time ago," said Team GB chef de mission Andy Hunt.
"If you are competing within 72 hours of the event, we ask you to think about the impact on performance. But every athlete is given the opportunity to march unless, as a team, they decide not to.
"We're going to have a good attendance, it won't be a full delegation but around 50 percent of the team will be there."
British athletes will be the last to march into the stadium as host nation, meaning they won't be involved until nearly midnight.
The proximity of the athletes' village will make logistics easier but many coaches are reticent for their charges to be involved, with ambitious forecasts predicting they won't be back in their beds until at least 1am.
However, organisers have trimmed the length of the parade of nations by restricting the number of non-competing officials who can march to just one per sport per team.
"No athlete goes to the Games who doesn't want to go to the opening ceremony," added BOA chairman Colin Moynihan.
"But if you know are are starting marching at 11.30 at night, you have to balance that with your competition schedule, performance always comes first."
UK Sport, who have spent over £500 million funding sports since London was awarded the Games in 2005, have set Team GB a target of at least 48 medals, one more than in Beijing, and a fourth place finish on the medal table.
"We have a real challenge to be able to deliver that. It's tough, the margins between gold and silver medals are narrow. The great excitement about sport is uncertainty and how tough it is," added Moynihan.
"But we'll be the best prepared and best supported British Olympic team ever.
"We are not a team of stars but a star team. It's been a long journey, seven years, but everything we do is athlete centric, performance driven and professionally run.
"The competition here is going to be incredibly tough. All the teams have spent as many hours as we have ensuring the preparation of their athletes for these Games.
"The delivery of results will be a major task in the light of the outstanding competition that we face."