The increasingly acrimonious election battle for South Thanet took another twist tonight when Labour likened Ukip to the BNP in a letter going out to hundreds of voters in the seat.

The letter is being depicted by the party as the latest stage of a charm offensive to win over voters, particularly women who have voted Conservative before.

But the comparison the letter makes between the BNP and Ukip has inevitably produced a row, with Ukip complaining of a smear.

The letter, signed by candidate Will Scobie and seen by the KM Group, refers to the campaign the party fought against the BNP in Barking, held by Labour's Margaret Hodge. She is due to visit the constituency next week.

Mr Scobie writes:

"The one thing that the BNP had in common with UKIP was the way in which they denigrated the area and the people living there" - Will Scobie, in the letter

"For me the highlight of the last General Election in May 2010 was when Margaret Hodge won her seat in Barking and sent the BNP (British National Party) into meltdown.

"Quite right too, some of the thugs that inhabited that party wouldn’t have been out of place in Germany in 1939."

It goes on: "The one thing that the BNP had in common with UKIP was the way in which they denigrated the area and the people living there.

"Something Margaret Hodge fought against and in the end managed to bring communities together and start to find solutions to the problems in Barking.

"So you may not be surprised that Margaret Hodge has accepted my invitation to come to Broadstairs and talk to local people about the election and some of the lessons she learnt from the last election.

"Margaret will tell you how on polling day in 2010 thousands of people came out to vote for her, not just because she was the Labour candidate, but because she was the one standing up for Barking, its residents, businesses, schools and communities."

It adds "And like Margaret when people think about South Thanet, I don’t want them to associate our area with the latest nasty comment from UKIP.

"We need an MP who will represent the area with diligence and pride, and who will defend and promote the constituency."

Ukip is sensitive to comparisons with far right organisations and moved swiftly to denounce the letter.

A senior Ukip aide said the letter was a “disgusting smear against millions of voters who are choosing Ukip at this election.

"It will not go unnoticed, especially by the swathes of former Labour supporters who are choosing Ukip as the patriotic working man and woman’s party."

“It is precisely smears like this that push Labour voters towards Ukip because they know there is only one party to thank for the demise of the BNP and the return of moderate democratic patriotism in this country.”

Mr Farage is due to hold a public meeting in the constituency tomorrow as the fight for South Thanet looks increasingly close in what will be a three-way fight.