Saints, who were crowned European champions in 2000, made too many errors throughout, which saw Castres secure their first Heineken Cup victory since last December – also against Northampton.

Hartley’s side went into the clash in Toulouse following a 24-15 win against Glasgow Warriors, and currently sit top of the Aviva Premiership.

And although disappointed with the defeat in France, 26-year-old Hartley believes the Saints should still be seen as contenders for the European trophy.

“We’re not out of it,” said Hartley, who featured in all five of England’s RBS 6 Nations matches earlier this year.

“We’ve got Ulster next at our place, and we’ll know where we are in terms of qualification a lot better after that one. There is still a long way to go in the competition.

“We knew that if we were to be considered contenders for the competition, we needed to win games at home against the likes of Ulster and that's where our focus now turns to.”

Saints can console themselves with a losing bonus point thanks to Stephen Myler’s 72nd minute penalty and Hartley revealed that was the goal at half-time, with his side trailing 13-6 at the break.

“We said at half-time in the Glasgow game that we would target the bonus point, and it was the same again here,” added Hartley.

“We were pleased to come away with a bonus point. Our defence was pretty good but we just didn't do the basics particularly well.”