Jill Seymour, who has held the West Midlands seat for Ukip since 2014, criticised the party's leadership under Gerard Batten, saying it had abandoned the centre ground and moved towards the far-right.

The Wellington-based MEP said she was proud of the role she played in helping Ukip grow under Mr Farage's leadership, but said it was no longer the party she joined.

“I am not walking away from the party’s original core principles, but the present party’s direction means it has walked away from me, and its original membership,” she said.

“I shall now be sitting as an officially designated Brexit Party MEP for the remainder of my mandate in the European Parliament.”

Her term of office was due to expire next month, following Britain's decision to leave the European Union, but the recent decision to postpone Brexit means that UK voters could be asked to take part in the European Parliamentary elections on May 23.

Mrs Seymour added: “It is a disgrace that we are even in a position to be talking about having European elections once again, but I would ask everyone who believes in democracy, and honouring the result of the 2016 referendum, to show the political establishment they are disgusted with the betrayal of Brexit by this Government, and go out and vote, believing in Great Britain and its independence.”

Mrs Seymour's husband Brian, who has been a member and donor to Ukip for more than 20 years, has also resigned from his lifetime membership of the party.

She said: “Under Nigel Farage’s leadership, Ukip gained brand recognition, and an army of stalwart supporters.

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“It was the hard work of the grassroots membership who helped me, and my colleagues, to persuade the British public to vote overwhelmingly for UKIP in the 2014 European elections.

“No one person should ever be bigger than a political party, but sadly I believe that Mr Batten will be instrumental in its demise.”

She accused Mr Batten of attempting to ‘bully’ her into leaving the Europe of Freedom and Direct Democracy group in the European Parliament, and follow him into the controversial Europe of Nations and Freedom group, which also includes the French far-right leader Marine Le Pen, the Austrian Freedom Party, and the Italy's League.

“It was suggested that, if I did not do this, it would be a black mark against me," said Mrs Seymour. "I do not appreciate being bullied."

Mrs Seymour resigned as the party's transport spokesman at the end of last year, but decided to remain in the party she was elected to represent.