Professor H.W.R. Wade in Administrative Law, 5th edition, at page 232 observes that the basic principle of estoppel is that a person who by some statement or representation of fact cause another to act to his detriment in reliance on the truth of it is not` allowed to deny it later, even though it is wrong. Justice here prevails over truth. Estoppel is often described as a rule of evidence, but more correctly it is a principle of law. As a principle of common law it applies only to representations about past or present facts. But there is also an equitable principle of ‘promissory estoppel’ which can apply to public authorities.
Promissory estoppel in Public law
In 1948, Denning, J. in Robertson v. Minister of Pensions, laid the foundation to the applicability of promissory estoppel in public law. As Prof. de Smith in his Judicial Review of Administrative Action, 4th edition at p.103 observes:
“There is a growing body of authority, attributable in large part to the efforts of Lord Denning, to the effect that in some circumstances when public bodies and officers, in their dealing with a citizen, take it upon themselves to assume authority on a matter concerning him, the citizen is entitled to rely on their having the authority that they have asserted if he cannot reasonably be expected to know the limits of that authority; and he should not be required to suffer for his reliance if they lack the necessary authority.”
The fact in Robertson’s case were these. The War Office wrote to Robertson, an Army Officer, who had claimed a disablement pension on account of the War injury, that his disability had been accepted as attributable to military service. But for this injury the responsible department was the Ministry of Pensions which the War Officer had not consulted. The Ministry later decided that the disability was not attributable and the Pension Appeal Tribunal upheld that decision.
In relying on the War Office letter the claimant had refrained from getting a medical opinion and adducing the other evidence which might have strengthened his case for such disability pension against the Ministry. On appeal to the Court, Denning, J. reversed the decisions of the Ministry and the Tribunal holding that the Crown was bound by the War Office letter and observe:
“The Crown cannot escape by saying that estoppels do not bind the Crown, for that doctrine has long been exploded. Nor can the Crown escape by praying in aid the doctrine of executive necessity, that is, the doctrine that the Crown cannot bind itself so as to fetter its future executive action”
It would appear that Denning, J. evoked two doctrines:
(1) that assurances intended to be acted upon and in fact acted upon; were binding; and
(2) that where a Government department wrongfully assumes authority to perform some legal act, the citizen is entitled to assume that it has that authority, and he dismissed the contention that estoppels do not bind the Crown by saying that ‘that doctrine has long been exploded’ and that the Crown cannot fetter its future executive action.
Professor Wade points out that the proposition about wrongful assumption of authority evoked by Denning, J. was immediately repudiated by the House of Lords in a later case in which Denning, LJ. had again put it forward in Howell v. Falmouth Boat Construction Company Ltd., L.R.  A.C. 837.
The Applicability in India
There appears to be a school of thought in India laying down that the doctrine of promissory estoppel applies to the Government except under certain circumstances. In Union of India & Ora. v. Indo Afghan Agencies Ltd  2 S.C.R. 366, Shah, J. speaking for the Court stated with approval the following observations of Denning, J. in Robertson’s case:
The Crown cannot escape by saying that estoppels do not bind the Crown for that doctrine has long been exploded. Nor can the Crown escape by praying in aid the doctrine of executive necessity, that is, the doctrine that the Crown cannot bind itself so as to fetter its future executive action. And the learned Judge held that this doctrine applies in India.
In Century Spinning & Manufacturing Co. Ltd. & Anr. v. The Ulhasnagar Municipal Council & Anr.,  3 S.C.R. 854, Shah, J. in remanding the petition to the High Court which it had dismissed in limine again observed:
“In Indo-Afghan’s case this Court held that the Government is not exempt from the equity arising out of the acts done by citizens to their prejudice, relying upon the representations as to its future conduct made by the Government. This Court held that the observations made by Denning, J. in Robertson’s case applied in India.”
The learned Judge observed that the court was not concerned with the principle which was dis-approved by Lord Simonds in Falmouth’s case and he added:
“If our nascent democracy is to thrive different standards of conduct for the people and the public bodies cannot ordinarily be permitted. A public body is, in our judgment, not exempt from liability to carry out its obligation arising out of representations made by it relying upon which a citizen has altered his position to his prejudice.”
In Motilal Padampat Sugar Mills Co.(P) Ltd. y. State of Uttar Pradesh & Ors.,  2 S.C.R- 641, Bhagwati, J. speaking for himself and Tulzapurkar, J. laid great stress on the facts that the principles laid down by Denning, J. in Robertson’s case were accepted by the Court in the Indo Afghan’s case but accepted the rejection of Lord Simonds and Lord Normands in Falmouth’s case of the extended principles enunciated by Denning, J. in Robertson’s case as Laying down the-correct law. But the learned Judge went down to say that this rejection did not mean that there could be no estoppel against the Crown or the public authority.
In Express Newspapers Pvt. Ltd. Ors. V. Union of India, (1985), the court was of the view that,
“In public law, the most obvious limitation and doctrine of estoppel is that it cannot be evoked as to give an overriding power which it does not in law possess. In other words, no estoppel can legitimate action which is ultra vires. Another limitation is that the principle of estoppel does not operate at the level of Government policy. Estoppels have however been allowed to operate against public authority in minor matters of formality where no question of ultra vires arises”.
 L.R., [19491 l K.B. 227]