The government has announced it is to undertake detailed planning in two possible locations for a new crossing of the lower Thames to the east of London.

In December, the government said it would discard one of three locations in the consultation and work on the remaining alternatives. Option A is close to the existing crossing, while option C connects the A2 and M2 to the M25 via the A13.

Before the decision is made on the site of the new crossing, route options are to be developed and appraised in detail. The government says this will enable the Highways Agency to assess the effect of the new remote payments system, Dart Change, which will improve driving conditions at the existing Dartford-Thurrock crossing after it is introduced from October.

Transport secretary Patrick McLoughlin said: "We are committed to a new Lower Thames Crossing, but whichever location is chosen, it will have a big impact on people in the area and we must make the right decision."

Mr McLoughlin insisted the work will not delay the introduction of the crossing, which is needed to reduce congestion.