The carriageway was closed between junction 15 near Northampton and Newport Pagnell Services, with congestion stretching back as far as junction 15a for Towcester.
Police were called to the incident at about 18:45 BST on Thursday and have also closed two northbound lanes.
A Northamptonshire Police spokesman said traffic was being turned around in an attempt to get it off the motorway.

Lasse Vibe netted a first-half double for the Bees which followed a Nico Yennaris close-range finish to make the result all but secure by half-time.
Zach Clough scored a second-half consolation from the penalty spot for the Trotters after Alan McCormack was penalised for a foul.
Bolton are 17 points from safety with six games remaining in the season.
Defeat for Bolton extended their dreadful sequence of no away wins in a year and means defeat at Derby at the weekend will spell relegation for Wanderers.
A second win in as many games for Brentford lifted them 12 points clear of the bottom three.
Alan Judge was the creator of the home side's first chance when he almost looped a cross from the right over Ben Amos, but struck the bar.
The midfielder provided a pinpoint cross from Vibe's second goal with a header, which was the Denmark striker's third goal in two games.
Brentford manager Dean Smith:
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"If you'd said to me three weeks ago that we'd have two wins on the spin at Nottingham Forest and again here, I'd have bitten your hand off.
"It means we can go to Ipswich on Saturday and try to make it three wins in a row and then we can be looking at a top 10 finish.
"I wanted more goals but with the run of form we've been on I wanted a result and that was the most important thing."
Bolton caretaker manager Jimmy Phillips:
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"We have to give the supporters something to cheer about rather than give them the kind of displays we gave in the first half tonight.
"In the second half we were more competitive and we always say we have to compete before we can play.
"Before the break we were nowhere near as competitive as we needed to be."

She launched an "Armpit Hair Competition" on China's popular micro-blog service Weibo to get women to take ownership of their bodies.
The competition attracted thousands of comments from Chinese women.
You have been sharing your thoughts via @BBC_HaveYourSay on Twitter. Here is a selection of your tweets:
@jen_vorhaut  in Washington DC, US, chooses not to shave her armpits.
@MARKDIDDY  from Nigeria and @Jenn_Abrams from the US share their reasons for preferring armpits to remain hairless.
But not all readers agree that shaving armpits is the best way forward. @bijubelinky  in London, UK, thinks it is down to the individual to choose.
@YHHSulami  from Manhattan, Kansas, US, thinks that shaving armpits is a way to keep clean.
@ennisorca is from Australia and says that shaving, or choosing not to shave, is a seasonal choice.
And finally, @WomenAlso_ from the UK raises the point that shaving armpit hair has nothing to do with hygiene.
Produced by Alison Daye
A lorry fire has closed a southbound section of the M1 in Northamptonshire.
[NEXT_CONCEPT]
Bolton could be relegated to League One on Saturday after they were beaten by Brentford at Griffin Park.
[NEXT_CONCEPT]
Xiao Meili is a well known women's rights activist in China.