Charlotte Browning has earned her first senior call-up for England.
Browning, who won national under-17s indoor titles at 800m and 1,500m last week, will compete in an international against Wales in Cardiff on Sunday.
She said: "I was originally going to run at under-19s but my winning times in the national indoors were faster than most senior girls so they've promoted me. It will be a tough challenge."
Browning, 16, from Chichester, missed out on a World Under-20 Cross Country Championship spot by two places.
She was seventh in the English Championships at Nottingham, only the first five qualify to represent Britain in Brussels next week.
Browning had been expecting to compete in her own age group and bid for what would have been a fourth successive title.
But she switched and was hoping to earn an under-20 vest.
She said: "I don't have any regrets even though I would have probably won at under-17s.
"I was pleased to go so close to getting into the team. I was delighted to last because I had not experienced running over 6km before."
Charlotte Best (Crawley) also just missed out on a World Under-20 Cross Country Championship place when she finished sixth at Nottingham.
Sundai Boulton, the Phoenix runner from Brighton, was 33rd in an under-13s race won by Elisha McColgan, daughter of former superstar Liz.
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