Hopes of a drama-free general election manifesto launch for the Conservatives have been dashed after protesters ambushed Theresa May's battle bus in Halifax, West Yorkshire, on Thursday 18 May.

The Unite activists unfurled an anti-zero hours contract banner in front of the vehicle not long after the cabinet had decamped from it.

"From exploitative zero and short hours contracts to bogus self-employment, the world of work has become increasingly insecure under this Tory government," a spokesperson for the union said.

"Bad bosses have been allowed to get away with ever more creative ways to exploit workers and dodge their responsibilities.

"With a shocking 3.8 million people experiencing in-work poverty it's high time that work in this country paid and the misery of insecure work was eradicated.

"Only Labour will act on the growing chorus of concern by banning zero hours contracts, strengthening workplace rights and promoting strong trade unions and collective bargaining as part of a package for decent well paid work."

The stunt came as May is expected to unveil the Tories' flagship policies, including capping energy prices, extending grammar schools and pledging to slash net migration to tens of thousands.

But the prime minister is expected to face resistance over axing the so called "triple lock" on state pensions, which make sure the retirement pots increase even when inflation is low. The £300 winter fuel allowance will also be controversially scrapped under a Conservative government.

The announcement comes with just three weeks to go before the general election on 8 June. The latest opinion poll from Ipsos MORI, of more than 1,000 people between 15 and 17 May, put the Conservatives 15 points ahead of Labour (49% versus 34%).