Baffled motorists are scratching their heads over a discount scheme they were told would give them free trips at the Dartford Crossing.

The Government has trumpeted its plans to give Dartford drivers concessions when the toll charges increase later this year. Discounts will also apply to drivers in Thurrock, Essex.

The proposals, announced by the Department for Transport in February, stated: "Local people will be able to make 50 free journeys in a year. Once the free journeys are used up, the charge will be just 20p per crossing."

But confused motorists have taken issue with the fact they will have to pay £10 each year to register for the discounts on offer. This is exactly the same as what it would cost to make 50 trips at the tolls, priced at the discounted rate.

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The fee, which a Department for Transport spokesman said was to cover administration costs, will register residents for an electronic tag similar to the Dart Tag.

This would be displayed in car windscreens and identify Dartford drivers at the tolls.

Neil Pearson-Coffey, of Southfleet Road, Bean, writes: "Now I may have missed something, but I was under the impression that to enjoy that service (50 free crossings) we would have to purchase a £10 Dart Tag.

"I'm no mathematical genius, so I'll leave it up to you to work out. How many journeys normally costing 20p can you get for £10?"

Motorists who only use the crossing occasionally would be better off without the discount.

For them, it would be more cost-effective not to register for the discount tag and simply pay the new charge, which is increasing to £1.50 for cars.

Kenneth Shepherd, of Stanham Road, Dartford, said: "If you are a frequent user it's all well and good, but not if you only use the tolls a few times a year.

"I only use it twice a year to go to Scotland, but I'm not going to pay £10. For me it's cheaper to pay £6 for the four trips."