* Q1 underlying sales down 0.9%

* First three weeks on Q2 underlying sales down 2.5%

* Cautious on market outlook but keeps profit forecast

LONDON, May 21 (Reuters) - European home improvement retailer Kingfisher reported lower underlying sales in its first quarter and the early weeks of the second, with resilience in core products offset by weakness in so called "big ticket" categories.

The FTSE-100-listed group, which owns B&Q and Screwfix in Britain and Castorama and Brico Depot in France and other markets, said on Tuesday it remained cautious on the overall market outlook for 2024 "due to the lag between housing demand and home improvement demand."

Nevertheless, Kingfisher kept its forecast for an adjusted pretax profit for 2024/25 of 490 million to 550 million pounds ($623-$699 million), down from the 568 million pounds made in 2023/24.

It said group like-for-like sales fell 0.9% in the first quarter to April 30 and were down 2.5% in the three weeks to May 18.

While like-for-like sales in core categories, such as tools and hardware, fell 0.3% in the first quarter they were down 6.3% in "big ticket" categories such as kitchens and bathrooms.

Like-for-like sales in seasonal products such as garden furniture were up 1.9% despite unfavorable weather in April.

Survey data published earlier this month showed British consumers kept a tight rein on their spending in April, which partly reflected dismal weather.

Shares in Kingfisher were down 0.7% in early trading, extending 2024 losses to 7.8%.

The group said it expects the UK & Ireland home improvement market to be at best "flat" in 2024, with the French market seeing a "low single-digit" decline at best.

The group's total sales in the first quarter rose 0.3% to 3.3 billion pounds, with like-for-like sales up 1.2% in the UK & Ireland and up 0.4% in Poland, but down 5.3% in France.

It said B&Q and Screwfix won market share in the UK, while in France, Castorama and Brico Depot performed broadly in line with the weaker market. ($1 = 0.7868 pounds) (Reporting by James Davey, Editing by Paul Sandle and Susan Fenton)