Duffy comes from the town of Nefyn in north-west Wales

Her success means that Leona Lewis's Sport Relief single, Better In Time, only entered the chart at number two.

Duffy's Rockferry was the number one album for a second week, with US rock band OneRepublic the highest new entry at two, with Dreaming Out Loud.

Meanwhile, Nickelback's Rockstar single held on to a top five placing in its 18th week of release.

Dance act H Two O featuring Platnum dropped from two to three, while OneRepublic held firm at four with Stop and Stare.

UK TOP FIVE SINGLES 1 Mercy - Duffy 2 Better In Time / Footprints In The Sand - Leona Lewis (pictured) 3 What's It Gonna Be - H Two O ft Platnum 4 Stop and Stare - Onerepublic 5 Rockstar - Nickelback Source: Official Chart Company

Other new entries in the Top 40 singles chart came from Panic At The Disco, Hard-Fi, Foals and Mancunian rockers Elbow.

Girl group Sugababes had the highest climber, as Denial rose 19 places from 34 to 15.

Leona Lewis's Footprints In The Sand appeared in the chart twice - once as the double A-side to her Sport Relief single, and again as a standalone track at number 25.

The X Factor winner scored a further placing in the rundown with her former number one Bleeding Love, which re-entered the chart at 40.

UK TOP FIVE ALBUMS 1 Rockferry - Duffy 2 Dreaming Out Loud - OneRepublic 3 All The Right Reasons- Nickelback 4 Spirit - Leona Lewis 5 Back To Black (Deluxe Edition) - Amy Winehouse Source: Official Chart Company

In the album chart, Nickelback remained third with All The Right Reasons, with Lewis's Spirit rising 12 to number four.

Amy Winehouse dropped three places to five with Back to Black.

Classic Soul Hits, by Motown legends The Temptations, was the second highest new entry at number eight.

Other new albums in the Top 40 included Oracular Spectacular by MGMT at 12, The Very Best of Clannad at 20, and Superabundance by New Knives at 28.