Any decisions on future schemes would be taken following the spending review.
Tolling the M4 in Wales would be a matter for the Welsh Assembly.
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Owners must also prove they have something to pick up dog mess with under new rules imposed by one council.
The former prime minister made the comments as he prepares to return to politics saying he wants to be "part of the debate" on Brexit.
The new measures would mirror childcare vouchers and see workers sacrifice some of their salary to benefit.