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Updated at 1547 GMT

By Khushi Singh

Sept 5 (Reuters) -Britain's benchmark FTSE 100 stock index ended lower on Thursday, weighed by a decline in pharma shares, while a handful of stocks traded ex-dividend, adding to losses.

The blue-chip index .FTSE closed down 0.3%, falling for the fifth consecutive session for the first time since May.

The domestically-focused mid-cap FTSE 250 .FTMC index lost 0.2%, after touching a nearly one-month low on Wednesday.

The pharma and biotech index .FTNMX201030 was the biggest sector loser, tumbling 2.9% as drugmaker AstraZeneca AZN.L sank 3.9%.

Police in China have detained five current and former AstraZeneca employees for questioning about potential illegal activities, Bloomberg News reported, citing people familiar with the matter.

Primark owner AB Foods ABF.L was the top FTSE 100 loser, down 8.5% after the company said it expected Primark's like-for-like sales to decrease by 0.5% in second half of the year due to the unfavourable weather in Britain and Ireland.

Insurers Prudential PRU.L and Admiral ADML.L, paper and packaging firm DS Smith SMDS.L, and chemicals group Croda CRDA.L fell between 1.6% and 3.3% on trading ex-dividend.

In contrast, the homebuilders index .FTNMX402020 was the top sector performer, rising 2.6% after Vistry VTYV.L announced a share buyback worth 130 million pounds ($171 million) following a rise in half-year earnings. Vistry shares rose 8.5% to the top of FTSE 100.

Among mid-caps, Alfa Financial Software ALFAAL.L rose 10.3% rise after saying it expected full-year revenue to be 1 million pounds ahead of previous expectations.

Online retailer ASOS ASOS.L jumped 18.1% after forecasting adjusted core profit at the top end of market forecasts.

On the data front, growth in Britain's construction industry slowed in August despite the biggest pickup in homebuilding in nearly two years, adding to signs of easing inflation pressures.

British companies expect to raise selling prices by the smallest amount in nearly three years but wage growth shows no sign of cooling, another survey showed.

Reporting by Khushi Singh, Ankika Biswas in Bengaluru; Editing by Varun H K and Mark Potter

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