Ed Balls will express his commitment to HS2 today, but will reiterate that Labour's support for the project is "not at any cost".
He is expected to tell the annual Confederation of British Industry conference today: "Labour supports HS2 and the idea of a new North-South rail link because of capacity constraints on the existing rail network. But our support for it is not at any cost.
"The Labour Party cannot - and will not - give the government a blank cheque."
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Five people were killed and dozens more were injured after a car and knife attack in the capital on Wednesday afternoon.
Among those expected to attend the ceremony will be former US President Bill Clinton and ex-Northern Ireland first minister Peter Robinson.
Five people have been killed and 40 injured after a car and knife attack in London on Wednesday.