White, playing as usual up the board in the diagram, can force checkmate in two moves, however Black defends.

This weeks puzzle is the opening round of a national contest where Guardian readers traditionally perform strongly. You have to work out how White, playing, as usual, up the board in the diagram, can force checkmate in two moves, however Black defends.

The puzzle is the first stage of the annual Winton British Solving Championship, organised by the British Chess Problem Society. This competition is open only to British residents and entry is free. The prize fund is expected to be at least 1200, plus awards to juniors.

If you would like to take part, simply send Whites first move to Nigel Dennis, Boundary House, 230 Greys Road, Henley-on-Thames, Oxon, RG9 1QY. Or by email to winton@theproblemist.org.

Include your name, home address and postcode and mark your entry Guardian. If you were under 18 on 31 August 2019, please include your date of birth.

The closing date is 31 July. After that, all solvers will receive the answer and those who get it right will also be sent a postal round of eight problems, with plenty of time for solving.

The best 20-25 entries from the postal round, plus the best juniors, will be invited to the championship final in February (subject to Covid-19 restrictions). The winner there will qualify for the Great Britain team in the 2021 world solving championship, an event where GB is often a medal contender.

The starter problem, with most of the pieces in the lower half of the board, is tricky and with an unusual twist. Obvious checks and captures rarely work. It is easy to make an error, so review your answer before sending it. Good luck to all Guardian entrants.

Magnus Carlsen survived some anxious moments this week in his quarter-final match in the online Clutch International before the world champion overcame Americas top junior Jeffery Xiong. The 19-year-old Texan had a purple period in the middle of the 12-game series when he had a run of five games with two wins and three draws.

Carlsen was dominant at the start and the finish and his best two victories were imaginative attacks where the rare knight move Nh7! featured.

The event, financed by the St Louis billionaire Rex Sinquefield who has made his home city a global chess centre, has the highest prize fund yet, $265,000 (approx 207,000), for an internet tournament.

Carlsen controlled the first session of his semi-final on Thursday evening as he led Armenias Levon Aronian 6-2 without losing a game. Wesley So also led 6-2 in the all-American semi-final against the world No 2, Fabiano Caruana.

A Carlsen v So final would be far from a done deal for the world champion, as So is currently in excellent form. The semi-final is also not over yet due to the Clutch scoring system where the final two games (of six) count double on the first day and triple on the second. In his interview after the Thursday session, Aronian declared his intention to go into berserk mode for the last six games, taking extra risks to get back into the match.

Both semi-finals can be viewed live online for free with grandmaster commentary, starting at 7pm on Friday.

Read more:

Chess: national solving championship opens for entries from Britain this week - The Guardian