* FTSE 100 down 0.6 pct, FTSE 250 down 1.3 pct
* Top EU court advisor says Brexit could revoke Article 50
* Thomas Cook hits 6-year low
* Gold stocks rally on higher prices
* Quarterly reweighting roils some shares(Adds details, closing prices)
By Helen Reid and Josephine Mason
LONDON, Dec 4 (Reuters) - Concerns about growth, doubtsabout a U.S.-China trade truce, and a rising pound droveBritain's top stock index down on Tuesday, while precious metalminers regained their lustre as gold prices rallied.
The FTSE 100 closed down 0.6 percent with thestronger sterling dragging on its exporter-heavy constituents.
Still, the market outperformed its euro-zone peers due togains in its heavyweight oil and gold mining sectors and aflight to defensive stocks, popular at times of economicturmoil.
But as investors' optimism on a deal between U.S. PresidentDonald Trump and Chinese President Xi Jinping to delay tariffsevaporated, concerns about Brexit returned.
The currency rose as much as 0.9 percent after an adviser tothe European Union's highest court said Britain should beallowed to unilaterally revoke its departure notice.
Investors were on tenterhooks as the British parliamentbegan five days of debates on Prime Minister Theresa May'sBrexit deal ahead of a crucial Dec. 11 vote.
"We're fairly neutral on the UK, but we're playing it mainlythrough passive exposure - because you get the benefit of thecurrency if it weakens," said Rory McPherson, investmentdirector at Psigma Investment Management.
The quarterly reweighting of the indexes also roiled someshares, with Hiscox expected to oust Royal Mailfrom the blue chips and storied carmaker Aston Martinset for promotion to the midcap index just two months after itsIPO.
Final moves will be announced by the LSE on Wednesday basedon market cap at Tuesday's close.
BAE Systems, down 5.5 percent, was one of thebiggest decliners after Deutsche Bank cut its price target onthe stock. Ferguson, previously known as Wolseley, lost4.3 percent after the world's largest heating & plumbingequipment supplier published first-quarter results.
While trading profit was in line with expectations, analystssaid the performance of the U.S. business, the company's largestmarket, disappointed.
Mining stocks such as Antofagasta and Glencore, down 3.5 and1 percent respectively, fell as copper prices eased back ondoubts over how fragile a trade truce struck at the G20 onSaturday was.
Randgold Resources and Fresnillo both rosemore than 3 percent as the gold miners benefited from investors'bid for gold, considered a safer store of value in volatilemarkets.
Gold prices climbed to over a one-month high as the dollarslipped.
Energy shares were a relatively bright spot, with oil majorBP rising 1 percent and Shell ending littlechanged. Both ended off earlier highs as worries over tradecurbed gains on crude oil prices.
Thomas Cook shares fell as much as 16.9 percent tohit their lowest level in six years amid growing concerns aboutthe tour operator's debts.
The shares closed 3.9 percent lower after an extremelychoppy session as investors braced for the stock to be demotedfrom the midcap index. Its credit default swaps soared and bondsfell to all-time lows.
The tour operator and travel company has lost 60 percent inthe past week since its second profit warning in as many months.
Shares in construction and engineering firm Kierfell 4.1 percent after Canaccord Genuity cut its rating on thestock to "hold" from "buy" after the company's recentlyannounced rights issue.
"After fees, a worse than previously assumed working capitaloutflow and confirmation of the average debt level using dailyaverages, the annualised benefit from the rights issue isarguably only around 90 million pounds," the analysts wrote.
Online financial trading firm IG Group, down 9.7percent, was among the biggest decliners on the FTSE 250 afterit forecast a drop in trading revenue.
Ted Baker fell a further 8.4 percent, extending itslosses from Monday when an employee petition criticising theCEO's policy of hugging sent the stock down 15 percent.
Consort Medical shares plunged 21.5 percent afterthe pharmaceutical firm's full-year pre-tax profit was dented byabout 3 million pounds due to delays with Mylan in their Wixeladrug programme.
The stock suffered its worst day in a quarter-century.
Overall, analysts are downgrading their estimates ofdomestic U.K. stocks' earnings at the fastest rate since theBrexit vote of June 2016.
"We feel there's better trades to be had than being exposedto U.K. domestics. It could be a fantastic trade if we get agood deal, but at the moment they're very beaten down," saidPsigma's McPherson.
(Reporting by Helen Reid; Editing by Angus MacSwan and AndrewCawthorne)