LONDON, Oct 4 (Reuters) - Britain's antitrust regulator said it would clear Hitachi's acquisition of Thales' signalling business after the Japanese company addressed its concerns by offering to sell part of its mainline signalling business.

The Competition and Markets Authority (CMA) had said the 1.7 billion euro ($1.8 billion) tie-up could harm competition because both businesses were leading suppliers of signalling for mainline and urban railways, along with Siemens and Alstom.

Hitachi responded by offering to sell its existing mainline signalling business in the United Kingdom, France and Germany, which the CMA said would ensure customers such as Network Rail would not be negatively affected by the merger.

The CMA also said on Wednesday that it was no longer concerned about competition in urban rail networks such as London Underground after considering new evidence.

Hitachi has made a similar offer to sell assets in France and Germany to address EU antitrust concerns.

The EU antitrust enforcer has set a Nov. 6 deadline for its decision.

($1 = 0.9563 euros) (Reporting by Paul Sandle; editing by William James)