Mark van Gisbergen landed the two late penalties which boosted Wasps' Guinness Premiership play-off hopes and left Northampton's relegation survival hopes hanging by a thread.

Despite scoring the four tries which ultimately secured a valuable winning bonus point, Wasps had to rely on Van Gisbergen's 75th and 81st-minute penalties for victory following a spirited Saints second-half fightback.

Wasps looked to be cruising to victory when Paul Sackey, playing his first match following a three-month absence with a knee injury, ran in his second try in the 55th minute to stretch their lead to 29-15.

But Northampton refused to give up without a fight and hit back with two tries which levelled the scores at 29-29 13 minutes later to earn a losing bonus point which could still prove crucial.

Hooker Dylan Hartley sparked the revival when he took a pass from scrum-half Mark Robinson to force his way over in the left-hand corner in the 58th minute to grab his reward for a hardworking display in the visiting pack.

Skipper Bruce Reihana converted from the touchline and was on target to add the points again when second-half replacement Christian Labit went over in the same corner for his side's touchdown.

Reihana missed with the first attempt but was handed a second chance after Wasps England winger Josh Lewsey was ruled to have charged his original kick too early.

The late scoring burst put Northampton in sight of an unlikely but crucial victory but they failed to capitalise.

Van Gisbergen, who secured victory over Saracens last weekend with an injury-time conversion, made sure Wasps were the beneficiaries of another late show.

Northampton had overcome the early loss of former All Blacks fly-half Carlos Spencer to lead 12-10 at half-time.

Spencer, who kicked Saints ahead with a third-minute penalty from 47 metres after Wasps were caught offside, damaged his left ankle two minutes later when he was tackled by Van Gisbergen.

He tried to play on but was finally forced to admit defeat and limp off in the 16th minute, to be replaced by Luke Myring.

While Spencer was receiving treatment, centre Reihana stretched a lead to 6-0 when Wasps strayed offside again.

The home side hit back to take the lead with centre Fraser Waters taking a perfectly-timed pass from fly-half Danny Cipriani to burst through the defence for a 13th-minute try, converted by Van Gisbergen.

They should have had a second when Sackey, returning from the injury which ruled him out of England's entire RBS Six Nations campaign, released a short pass for Cipriani to squeeze in at the right-hand corner flag but the score was disallowed because the fly-half grounded the ball on the touchline.

Northampton were battling for their Premiership lives and they made Wasps pay for the penalties they conceded at the breakdowns, with Reihana landing two more in the first half, to one by Van Gisbergen.

But the Saints failed to capitalise fully on their extra man advantage after Lewsey was sin-binned in the third minute of injury time, with Reihana missing two more penalties in first half injury time which eventually proved costly.

Wasps introduced England flanker Joe Worsley for Dan Leo at half-time and the switch provided almost instant dividends.

Five minutes into the second half Worsley powered through the middle to set up the move which saw Waters combine with centre partner Dominic Waldouck who passed back inside for Sackey to race in for his first try.

Van Gisbergen converted with a kick which went over via the crossbar and a post to put Wasps 17-10 ahead.

Lewsey returned but Northampton had England left-wing Ben Cohen yellow-carded for illegally halting a Wasps' attack from an offside position.

Hartley appeared fortunate not to join him in the sin-bin for a late challenge on Eoin Reddan which clearly incensed Wasps' stand-in skipper Phil Vickery.

Wasps immediately capitalised on their superior numbers with Cipriani taking a short pass from Redden to dummy his way through the Northampton defence for a 49th-minute try and Van Gisbergen drew the cover to send Sackey in for their fourth try six minutes later.

Northampton came again to level the scores but their hopes were dashed by the two late blows struck by Van Gisbergen.