Their colleagues think it's "stupid" and "ludicrous", but some MPs have in recent years taken to voting both yes and no in the Commons as a means of registering an abstention. Why?

It was back in the 1970s that Commons Speaker George Thomas warned MPs considering a stroll through both the "aye" and the "no" lobbies - where MPs get their votes counted - not to make a "nonsense of our procedures".

In their own words: MPs who voted for and against gay marriage Angie Bray: "I am not implacably opposed to change, I need to be convinced that it is necessary and has been properly thought through." Andrea Leadsom: "I find myself genuinely torn... I cannot vote against a measure that would mean so much to the minority of homosexual couples for whom marriage is the ultimate recognition for their genuine feelings for each other. Yet nor can I vote for a measure that risks centuries of faith based belief in marriage." Phillip Lee: "I gave this issue deep thought and concluded that the legislation before me was not worthy of my full support. I did not, however, want to give any impression that I was against a formal recognition by society of committed homosexual relationships." Charlotte Leslie: "I felt it was the only intellectually and morally honest response... I cast a vote in favour of the bill as I do not oppose the fundamental principle behind it.... However, I also cast a vote against the bill as I believe it has serious flaws." Rob Wilson: "The government's bill is rushed and in my view there are a number of reasonable concerns that need to be dealt with... we shouldn't just wave this through."

"All of us who are members of this House are transient trustees of the dignity of the House," the Speaker said. "I look to all right honourable and honourable members to support me in maintaining respect for our traditional system of voting in this House."

Two MPs had decided to vote in both lobbies on clause one of the then Labour government's Scotland Bill, which asserted that devolution would "not affect the unity of the United Kingdom".

While not unprecedented, this bid to be seen as "positively abstaining" was so novel that it prompted much soul-searching in the days that followed, culminating in Speaker Thomas's ruling.

His successors have not deviated from disapproval of the practice, yet they have been powerless to stop it. Voting analysis website the Public Whip documents hundreds of instances of voting both ways since 1997, when its records begin.

"I think the present system of unofficial abstention is ludicrous," says Labour MP Chris Bryant.

Conservative MP Michael Fabricant contends that "trying to demonstrate that you were present but you don't have any view is a bit stupid".

But five of his Conservative colleagues voted both yes and no on the government's same-sex marriage legislation in February. "Why wouldn't you have an opinion about that?" wonders perplexed Tory Peter Bone.

The abstainers feel boxed in by the blunt instrument of Commons procedure, and believe they are expressing nuanced views - not no views.

'Disastrous'

Lib Dem MP Mark Williams voted yes and no on the government's proposed sell-off of publicly owned forests in 2011.

Voting in the Commons Commons votes are called "divisions", because MPs physically separate according to which side of the argument they are on.

MPs trundle through a corridor, or "lobby", behind the government's side of the Commons chamber to register a yes vote, and one behind the opposition's side to register a no - hence the Speaker's cry: "The ayes to the right, the noes to the left."

Some MPs are deployed as "tellers", who tick the names off as MPs walk through, then count the votes and announce the result. The whole process takes about 15 minutes.

He disapproved of the policy, but felt that as a member for Ceredigion, a constituency on the Welsh coast, it would have been wrong for him to take a stand on something that affected England only.

His Lib Dem colleague Mike Crockart also voted yes and no on the forestry sell-off.

The Forestry Commission's headquarters are based in his constituency of Edinburgh West, where he feared the impact of the policy would be "potentially disastrous".

"My deliberate abstention there was making the point that this is a big issue for me, that this needed to be looked at much more closely when it comes up again or you're not going to have my support, if it hasn't changed," he says.

Mr Crockart attributes the recent announcement of a new body to hold England's forests in trust - a "much, much better" idea, he says - partly to the pressure of government backbenchers, like himself and Mr Williams, deciding to abstain.

But why not simply vote against disastrous policies?

Abstention is "a different level of rebellion", Mr Crockart argues; if an MP constantly rebels against their party leadership, they will find that each individual act of rebellion has less impact. "You have to pitch your rebellion at the right level, and pick how often you rebel at the right level for them to continue to have an effect."

Mr Crockart also chose to abstain on a particular amendment to the Succession to the Crown Bill that would have removed the bar on Catholics becoming head of state.

Statistical outlier: The great Tory fisheries abstention of 2000 While the number of abstentions per year has tended to increase since 1997 (see graph), there was an extraordinarily high abstention rate in the year 2000. This is largely attributable to a single event. MPs had been asked to back the Labour government's assertion that fish stocks had "fallen to particularly low levels" and support its "intention to sustain stocks for the future". About 80 Conservative MPs - including then leader William Hague - used the new system of "deferred divisions", where MPs mark their votes on ballot papers instead of trooping through the lobbies, to register an abstention. "The paper for deferred divisions is quite simple to fill in. Members are highly experienced in matters of voting and there is no reason why any voting papers should be inadequately filled in or spoiled," Speaker Michael Martin said at the time. It would prove to be a one-off. "To ensure that the House's reputation is not damaged," the Speaker ruled, "in future, the name of any member who marks both boxes in a particular question in a deferred division should not be recorded as voting".

"I am actually a humanist, so I was trying to make the point that the succession to the crown should not be reliant on religion at all, any religion," he says.

Again, he didn't feel strongly enough about the subject, as he had done on raising tuition fees at English universities or some aspects of the coalition's welfare reform programme, to defy the party whips completely.

"You choose your battles," he says, "because otherwise it starts to get really quite difficult for you."

The MP who tabled that amendment on succession, the Conservatives' Jacob Rees-Mogg, believes it is "improper to vote in both lobbies to record an abstention".

"I think our job is to make up our mind on what is before us and therefore if you're abstaining you haven't managed to do that."

But there was one occasion on which he felt compelled to vote both yes and no himself, on a closure motion designed to curtail debate on whether or not the Youth Parliament should be able to use the Commons chamber.

"I don't like closure motions," he says, and had planned to vote against it.

"But the overwhelming will of the House was clearly for a closure motion." Worried that the "yes" camp might just fall short of the 100 votes it needed to render the vote valid and thereby lose on a "technicality", he decided to help nudge those on the opposite side of the argument over the finishing line.

Commons procedures do recognise certain situations where an MP might legitimately vote both yes and no: if they have mistakenly voted in the wrong lobby, for instance, they can mitigate the damage by voting the other way too.

Some MPs should be "a little more alert about which way they're going", Mr Williams asserts.

But Mr Rees-Mogg confides: "I know it sounds like a really dopey thing to do, but it's quite easy to go into the wrong lobby."

'Contradiction in terms'

One problem with the current system is that it is not always obvious whether an MP who has voted yes and no has made an innocent mistake or is taking a principled stand.

So why not tweak the system?

Abstaining - the traditional way Traditionalists argue that the best way to abstain is not to vote yes and no, but rather neither. This approach is not without its risks. Soon after becoming Lib Dem leader in 2008, Nick Clegg called on his MPs not to participate in a vote on whether to hold a referendum on the Lisbon EU Treaty. Three of his top team were so keen to express a view that they resigned their posts in protest. Theresa May, then shadow Commons leader, declared that Mr Clegg had "flunked his first big test" as party leader. He had "sat on the fence as a point of principle [and] divided his party after only a few weeks", she said. Then PM Gordon Brown concluded: "There is not much principle in recommending abstention."

Mr Crockart, an enthusiast for a move to formalised abstentions, recognises that there are barriers to reform: "Although it seems like a small thing, actually it would require quite a large change in the way that the House of Commons regards voting.

"You would really need to move to a system of electronic voting, but people don't want that necessarily because so much work goes on in the lobbies; it's the one time of the day that you can guarantee that ministers will be there so you can nobble them for 30 seconds."

An electronic voting system incorporating abstention works well in the Welsh Assembly, notes Mr Williams.

Although the "drama of having the results of divisions shouted out would be lost", he believes that analogue Commons votes can be a "tremendous waste of time" and the procedure committee should seriously consider backing a new voting system.

Until such changes are pushed through - and they would be sure to face potentially insurmountable opposition from the likes of Mr Rees-Mogg - there remains a certain logical force in Speaker Thomas's 35-year-old ruling: "If the name of any honourable member is recorded... as having participated in a division, he has certainly not abstained.

"That would be an obvious contradiction in terms."