French police can't arrest a British teacher who ran away with a 15-year-old school girl.

Megan Stammers and Jeremy Forrest, 30, were last seen going on a ferry to France last Thursday, local time.

Despite the age difference, it has been revealed it would be illegal for French police to arrest Forrest because he isn't a criminal suspect, the Daily Mail reported.

Prosecutor Jean-Philippe Joubert said Megan was old enough to travel voluntarily with Stammers, as the age of consent in France was 15.

Unless her parents complained of a kidnapping or she was being treated as a missing person, they would not get involved.

"In these circumstances, the couple will only be stopped if they are spotted during an identity check on the road, or if someone recognises them from photos," Joubert told the Daily Mail.

The use of CCTV footage has been minimal by French police, and they are unlikely to track Forrester's credit cards to work out where he is.

The pair had been exchanging hundreds of messages of love for at six months on Twitter, discussing how they would run away together.

Forrester, who is married, said he "only tweeted for one person," the Telegraph reported.

In June, the amateur musician said: "Some things are worth fighting for..."

"I've been trying to make sense. I've been shouting under my breath. How's any of this in my interest?" he said.

Megan, in one message, told Forrester "too many things keep me awake!!! Are you not sleeping either?!"

On one occasion, she repeatedly apologised to Forrester for her behaviour the previous evening, saying she was embarrassed and had a terrible headache, and asked if he hated her.

In late-June, Megan tweeted: "I just want to runaway forever," and Forrest answered: "Me & you. :-) Let's just run away."

Days later, she said: "I just want to get on a train / in a car and go somewhere with you."

Forrest said: "I want to go everywhere with you baby, & for the record you never look anything less than absolutely beautiful ..."

Megan also wrote a bucket list of wishes on a blogging website, which included falling in love and having someone write a song about her - both of which had been crossed off.

Other wishes included learn how to drive, learn how to skateboard, dye her hair blue and get her nose pierced.

Friends have claimed they alerted teachers at Bishop Bell School in Eastbourne when they were seen holding hands on a flight home from a school trip to Los Angeles in February.

Stammers' parents have denied knowing about their daughter's relationship with the married man, despite the head teacher, police and the local council investigating.

- © Fairfax NZ News