The Grand National is set to be hit by travel chaos after rail guards said they would go on strike.
Trains services were disrupted by a strike last month
The decision follows the failure of talks to resolve a dispute over hours.
The Rail Maritime and Transport (RMT) union said guards on Merseyrail will walk out on Friday and Saturday after a long-running row over a 35-hour week.
The union represents about 170 guards on Merseyrail, which runs services throughout the North West, including the station close to Aintree.
RMT guards claim that the changes to their working patterns could see them losing four "rest" days a year.
Bob Crow, the union's general secretary, said: "It is a nonsense for the company to go through the motions of mediation with its fingers crossed behind its back."
Merseyrail spokesman Rudi Boersma said: "We're very disappointed. The Grand National weekend is our busiest weekend of the year, but we have to recommend that people seek other means of getting to Aintree.
"We introduced a 35-hour week because the staff wanted it. We asked for some flexibility with rest days and the drivers and ticket inspectors agreed, but the guards want the 35-hour week without any conditions."
Merseyside Police advised racegoers to leave plenty of time to get to Aintree on both days, and use local bus services and taxis rather than their own vehicles.
On Good Friday, services suffered disruption when guards went on strike.
The Grand National meeting runs from Thursday till Saturday.