An enthralling World Cup has drawn more fans to the sport.

Television audiences are rising, and the sponsors are following in their wake. Star players are crossing the world for huge salaries.

If you are the kind of investor who likes to get in on the ground floor, you could do a lot worse than start investing in some rugby clubs.

But hold on. Just as the world's second-most-popular ball game is heading down the big-money route, the most popular . . . soccer . . . is having second thoughts.

That should prompt a rethink among rugby's ruling bodies. In small doses, money and attention could put rugby up there with other world sports. Too much, however, could ruin it, the way they have started to hurt soccer.

Still, there is little doubt that money is starting to flood into the game. The annual payments from England's Premier Rugby Ltd to its 12 clubs jumped to �30.6m in 2006 from �7.9m in 1999.

Where is it coming from?

Television, of course. British Sky Broadcasting, which has poured a fortune into soccer, has invested heavily in rugby as well. There are more sponsors, paying more. Plenty of top players, particularly from countries such as New Zealand and Australia, are now being tempted to play in Europe.

By some measures, international rugby is now the third-most valuable sporting franchise in the world.

According to its organisers, the 20-nation World Cup, which has been played every four years since 1987, now draws more TV viewers than any sports event except soccer's World Cup and the Olympics.

In a world where all sport is exploding in value, rugby is one of the few under-exploited games. It has a growing fan base and a fine tradition.

Nations such as Argentina and Fiji are making their mark on the game with places in this year's final knockout matches. Given their preference for oval-shaped balls, maybe even the US will get more serious about rugby one day. It probably has a better chance than soccer of taking off there.

And yet, soccer provides a warning as well as a guide.

That sport has been spoiled as much as enhanced by money.

Sepp Blatter, president of soccer's ruling body Fifa, is now trying to turn the clock back, at least by a few minutes. He has proposed that soccer clubs be forced to field at least six domestic players as of 2010 "to protect the local identity" of teams. Some British clubs now have only one or two native players in them; the big-money Spanish league is going the same way.

The trouble is, Blatter's proposal may be contrary to EU law, so he will have a fight on his hands. Blatter should persevere. The international and commercial appeal of soccer hasn't necessarily helped the game. There is a balance to be struck here, and rugby should think hard about how to get it right.

In fairness, one reason rugby is rising in popularity is that the standard of play has improved since the game went professional following the 1995 World Cup. Amateur sport can be exciting for its passion and commitment, but that soon wears off if the players aren't much good. A lot of missed passes are no fun to watch. Excellence requires hard work, and that doesn't happen unless people are well paid.

Likewise, a few world-class foreign players in a team can raise standards. Everyone benefits from playing alongside the best.

That said, the pitfalls are obvious.

In rugby, teams are now at risk of becoming little more than mercenaries without personal connections or commitment to the localities they represent. As in soccer, fans may be thrilled by the skills of the imported players at first.

Over time, they may wonder if they are just watching a brand, rather than the champions of their local city.

National teams, the main focus of rugby, risk decline as native players get squeezed out of local clubs. That has clearly happened to the England soccer team: few English players get to play in the Premier League, so the national side ends up with too few men to choose from. A weakened national team will mean falling support for the game.

Finally, as the money at stake explodes, billionaire investors are likely to move in, as they have moved in to soccer. If they buy all the best players, spending enormous sums to ensure success, there won't be much of a contest left. It has already happened to English and Spanish soccer, with only about three teams in either country having any chance of winning anything. If sport just becomes a clash of chequebooks, you can't expect the fans to stay interested.

A completely free-market approach doesn't work in sport. Why? Because you have to provide fair and open competition between lots of teams.

And you have to take account of local and national passions. Rugby is becoming more popular because it has an honesty and soul that money-obsessed soccer is fast losing. It would be foolish to put that at risk now, particularly when it is making up ground on its century-old rival.