The families of British athletes competing at the London Olympics will be guaranteed tickets after a landmark agreement yesterday between Games organisers and the British Olympic Association (BOA).

Further discussions will be held in the next fortnight on the number to be provided from the national allocation in an effort to avoid forcing the parents and siblings of Olympic competitors on to the black market.

The agreement, struck on the eve of completion of the first official Games venue in Weymouth today, is a response to problems over ticket allocation in China. The squeeze on supply led the parents of Rebecca Adlington, the double Olympic swimming champion, to a rogue website where they were conned out of £1,100. Five people behind the widespread scam were arrested this week by serious fraud investigators.

The small velodrome meant that key members of the British cycling set-up could not get access; the parents of Chris Hoy, the triple gold medal-winner, watched their son perform only after receiving last-minute tickets from a sponsor. The demand in London is expected to be even greater.

“Our first priority is an allocation of tickets to the athletes,” Colin Moynihan, the BOA chairman, said. “The principle has been agreed. The athletes have given their lives to be members of Team GB and their family should be given the opportunity of enjoying the Games around them.” The ticketing issue was hotly discussed at a formal Beijing debrief in London this week. The organisers want to avoid an embarrassing repeat of empty seats in Beijing that occurred despite its billing as the first sold-out Games.

The London Organising Committee of the Olympic Games (Locog) is working on ways to fill vacant seats with true supporters. One option is a Wimbledon-style system to re-use tickets held by spectators leaving early. It will also allow ticketless members of the public on to the Olympic Park in order to create the best possible atmosphere.

In Weymouth, where the upgraded sailing facilities for the Games will be unveiled by Paul Goodison, the gold medal-winning sailor, there is a push to stage the ten medal races within the harbour to give spectators a better view of the action.

“There is a natural amphitheatre at Newton's Cove and we'd like 10 to 15,000 people to be on the hillside, eating ice-cream and watching the big screens to see what's going on in London, but there are logistical barriers including security,” Mark Stubbings, the chief executive of the Weymouth and Portland National Sailing Academy, said.

“Sailing has never been the best spectator sport - the harbour may not be the best place from a pure aesthetic sailing point of view but it is from a spectator standpoint. We are just saying to Locog, don't lose sight of that.”

Construction workers have finished 150 metres of new slipway under a £6.5million project by the Olympic Delivery Authority (ODA). British sailors expect a better all-round test of their skills in Weymouth compared with Qingdao, the Chinese coastal city that was plagued by light winds and strong tides. Weymouth was chosen for its prevailing southwesterly wind - with an average speed of 16 to 17 knots - and a low tidal range of two metres.

ODA chiefs claimed the legacy of 2012 had already begun in Dorset. “From an economic point of view, this is a catalyst for more facilities,” Ralph Luck, the director of property, said. “Teams from across the world can come and train here, so it will affect the economy much more by being delivered early.”

Sailing requires two test events, so 2010 and 2011 will be important years for the Academy to demonstrate its potential as the ultimate Olympic venue. But there are still big logistical challenges: Weymouth takes three hours to reach by train from London and is accessible by only one main road.