Watford MP Richard Harrington “assumed” Baroness Thornhill would step down as Mayor of Watford after she was appointed as a Liberal Democrat front-bench spokesman.

Elected Mayor Dorothy Thornhill, who earns more than £66,000 in her role at Watford Borough Council, was one of 45 people to be appointed as a peer last Thursday.

But Watford MP Richard Harrington questioned whether Mrs Thornhill could work both as Mayor and the Liberal Democrat spokesman for Communities and Local Government.

Mr Harrington said: “I would like to congratulate her on becoming a member of the House of Lords but I naturally assumed with her becoming a front-bench opposition spokesman for a huge government department that she would be stepping down as Mayor.

“It is a full-time job being Mayor of Watford and it is a big job as an opposition spokesman for a huge government department. I am not decrying either job, but I don’t know how you can do both.”

Mrs Thornhill said she will not be “running up to the House of Lords” and insisted she will not start sitting in the upper house of Parliament until she has finished her fourth term as Mayor.

She said: “It is an honour. It does not mean I will be running up to the House of Lords. It is something I will do when I retire in 2018.

“I made that clear when Nick [former leader Nick Clegg] offered me the peerage.”

Speaking of her appointment as the communities and local government spokesman for the Liberal Democrats, Mrs Thornhill added: “It is an extension of what I have been doing with the Local Government Association.

“I am just a spokesperson. It does not mean I will be travelling up to Westminster every week. There will be times when I have to do other things, but I do that now in my role with the Local Government Association.”

Baroness Thornhill does not receive a salary or any remuneration for the role in Tim Farron’s team.

Members of the House of Lords who do not hold a position in government or key positions in the house do not get paid a salary. But they can claim a daily allowance of £150 or £300 when they attend the House of Lords.

Labour councillor Matt Turmaine, who stood against the Mayor in this year’s General Election, questioned the appointment if she was not going to sit in the House of Lords until 2018.

He said: “What is the point of appointing someone to the House of Lords and making someone a spokesman in an area of local government if you are not going to do it?

“Taxpayers are paying for you to do your Mayoral job but taxpayers are also paying you to sit in the House of Lords and you are not going to do any work.

“Watford has got a lot of changes at the moment, in terms of development in the High Street, and the proposals with the Health Campus and the housing shortages in the area and all of these things need a lot of attention over the next three years and that is presumably why she is saying she won’t sit in the House of Lords for three years. But I just don’t buy it.”

Mrs Thornhill has said she will not standing for re-election in 2018.

Mrs Thornhill is the second Liberal Democrat General Election candidate to be appointed to the House of Lords after an election defeat in the same year.

In 2010, Sal Brinton was appointed to the House of Lords after losing to Conservative MP Richard Harrington.