GEORGE Osborne will today unveil a plan to “get Britain building” again with a new National Infrastructure Commission to advise on what major projects the country needs and to hold Ministers feet to the fire if they fail to deliver them.

The Chancellor’s major announcement – an idea originally put forward by Labour’s Ed Balls - that could determine the future of projects like the £50 billion HS2 rail link comes after the Tories’ annual conference in Manchester was the focus of a major anti-austerity demonstration.

MPs, party members, journalists, lobbyists and staff at the conference centre were called "scum" as they ran the gauntlet of abuse from chanting demonstrators outside the Manchester Central venue. Several arrests were made.

The ugly scenes followed a peaceful protest, involving some 60,000 people.

The demonstration occurred as David Cameron resisted pressure, including from inside his own party, to back down on tax credit cuts. The Prime Minister made clear there would be no review of the cuts in next month's Autumn Statement, saying moving to an economy where people got paid more but were taxed less rather than paying more in tax and getting the money back in tax credits was “a better system”.

Ahead of his keynote speech, Mr Osborne unveiled a proposal to give working grandparents a new legal right to take time off to help care for their grandchildren.

The Chancellor said that many grandparents played a "central role" in caring for younger generations and the move would help them do that while allowing them to remain in work.

“Research shows two million grandparents have either given up a job, reduced their hours or taken time off work to look after their grandchildren. Allowing them instead to share leave with their children will keep thousands more in the workplace, which is good for our economy," he explained.

The plan will involve extending the current system of shared parental leave to cover grandparents as well as the child's mother and father. The total of 50 weeks’ leave will not be extended but the aim is to give greater flexibility to families in the first year of a child's life.

Meantime, Mr Osborne’s new infrastructure commission will be chaired by Labour peer Lord Adonis, who will resign the Labour whip and become a Cross-bencher; a move Treasury sources said was not meant to embarrass Labour leader Jeremy Corbyn but to show the new body was “apolitical”.

Asked why the Chancellor had lifted the proposal from Labour’s manifesto, one senior source replied: “The Chancellor is open to good ideas.”

He also made clear that the commission would not look at the expansion of Heathrow Airport as recommended by the Davies Commission, the response to which is expected from Mr Cameron before Christmas.

In his speech announcing the new commission, Mr Osborne is expected to say: “Where would Britain be if we had never built railways or runways, power stations or new homes? Where will we be in the future if we stop building them now?

“I’m not prepared to turn round to my children or indeed anyone else’s child and say: ‘I’m sorry we didn’t build for you...We have to shake Britain out of its inertia on the projects that matter most.”

The commission will be set up immediately and work over the long term. It will form part of a four-part plan, involving new planning rules in England to help building on brownfield sites and the ability of local councils south of the border to pool their pension funds to create wealth funds for investment in new infrastructure.