Britain's prime minister Tony Blair may not be taking sides (yet) but for Spears, Agassi and friends all eyes, surely, will this week turn to that great sporting arena: Committee Room One of the House of Lords.

It is here, with 500 million (HK$6.81 billion) at stake, that Agassi and his lawyers will contest the final set in the biggest match of their lives: Agassi v Robinson (the Inspector of Taxes.)

Victory means Agassi will escape paying 27,500 to the British taxman.

More important, Team Agassi estimates its stand could result in a whole galaxy of stars demanding the Inland Revenue return a total of up to 500 million in taxes, paid since 1988.

Spears, Woods and the Williams sisters are among the financially astute stars thought the most likely to demand their money back. Others may follow.

"This covers all the overseas entertainers who come to the UK to perform: sportsmen, football players, pop stars," explained Mike Warburton, a senior tax partner at global financial consultancy Grant Thornton.

"All their agents will be looking at this, Britney's included," he said.

As Spears may, or may not, know, this is all because of section 555(2) of the Income and Corporation Taxes Act 1988 and Thriller by Michael Jackson.

"It's all Michael Jackson's fault," explained Warburton. "He started this. The law was introduced following a series of Michael Jackson concerts.

"Overseas entertainers were making vast profits in the UK, and the British taxman wasn't getting enough." Hence the 1988 Act, section 555(2) obliging promoters and sponsors to deduct tax before paying stars for work performed in Britain.

For a while, the taxman was happy - very happy, because he wasn't just getting money from deals with British- based companies. If, say, a certain American tennis star had sponsorship deals with Nike, a US sportswear company, and Head, an Austrian sports equipment manufacturer, the British Revenue was entitled to some of that cash too.

Why? Because, Agassi, for a proportion of the tax year was earning his sponsorship money while playing at Wimbledon. (Time spent in leather catsuits at Wembley Arena was taxable too, Britney.)

So Agassi took up the fight for multimillionaire entertainers. He had a disastrous first set, his arguments smashed all over the High Court by Justice Lightman in March 2004, but recovered brilliantly in the Appeal Court.

Now, it's all down to the third set, the House of Lords appeal by the Inland Revenue.

Dispensing with serve and volley, Agassi is relying on the old "principle of territoriality" tactic.

If "territoriality" applies, he owes Brown nothing because Agassi Enterprises, his company, and his sponsors Nike and Head are outside British territory with no "tax presence" in the UK.

Overseas stars who entered into similar deals with foreign companies would be lining up to demand their money.

"Over the years, a lot of people have paid tax," Warburton said. "Now they want to reclaim it."

The stakes, then, are high, and the pre-match interviews brief.

"I'm sorry," said a spokesman for Revenue and Customs. "We are going to say absolutely nothing."

Perry Rogers, Agassi's agent, said: "We are waiting for the process to play itself out."

THE SUNDAY TELEGRAPH