THE X Factor charity song Hero burst into the charts at number one last night after becoming the fastest-selling charity single of the past 10 years.

The song is tipped to become the biggest-selling release of all time.

It has sold 313,244 in its first week of release – outselling the rest of the top 10 combined.

Among the millions listening to the countdown was Marisa Billitteri, 20, from Girlband whose powerful vocals can be heard on the Mariah Carey cover by the 12 acts which made it to the live finals of the TV talent show.

Yesterday, on the same night as the final of the Junior Search For A Star competition, which Marisa, from Kingswood, won in 2007, she said: "I can't believe we're number one, it's amazing news.

"It's pretty strange to hear my voice on the radio, it all feels a bit surreal.

"But it's a really good feeling, and it's all for such a good cause."

She heard the news on Radio One, which she was listening to with her boyfriend's family at his house.

She said: "I've spoken to mum and dad, they've been listening too and are really pleased. I can't believe how many records it's sold."

Marisa and the rest of Girlband are off to London on Tuesday to film the video for the single.

Profits from Hero will be split between The Royal British Legion's Poppy Appeal and Help for Heroes, a charity that provides support for wounded servicemen and women.

Woolworths has announced that its profits from sales of Hero will go to Help for Heroes. The high street chain has sold more than 50,000 singles.

Alan Hunt, head of music at Woolworths, said: "It's a great single and a fantastic cause that we are proud to support."

On Wednesday Chancellor Alistair Darling said VAT on the sale of the single will be waived.

HMV spokesman Gennaro Castaldo said earlier in the week: "Only Leona Lewis's debut hit A Moment Like This, which sold over 500,000 cop- ies in its first week, can compare."

Even Prime Minister Gordon Brown was getting in on the act, promising to buy his wife Sarah a copy for her 45th birthday, which was on Saturday.

Girlband has disbanded since leaving X Factor, but reunited to record the hit single.

The track costs 79p to download from iTunes, or £3.99 to buy in CD form.