A bottle of inflammable liquid was used during the attack in Stockbridge Village last week.

The vehicle had been used to help carry out regeneration work within the Spruce Grove area of Stockbridge.

Merseyside Police are questioning five men in their 20s from Stockbridge Village and a 24-year-old man from Dovecot.

Local contractors are hoping the incident does not mean the return of protection rackets by rogue security firms which have blighted the city’s construction sites.

Merseyside Police led a two-year blitz starting in 2006 known as Operation Seahog against security firms threatening contractors.

Nearly 1,000 sites were visited during the campaign which saw more than 50 private security firms investigated.

One local contractor told the Enquirer: “This was a massive problem a few years ago but the police operation certainly improved things.

“No-one wants to see a return to those days but this arson attack certainly smacks of tactics used in the past to scare contractors into paying for protection.”